The Corrosion and Biofouling Characteristics of Sealed vs ...

The Corrosion and Biofouling Characteristics of Sealed

vs. Perforated Offshore Monopile Interiors:

Experiment Design Comparing Corrosion and Environment Inside Steel Pipe

by

Monica M. Maher, P.E.

A thesis submitted to the College of Engineering and Science of

Florida Institute of Technology

in partial fulfillment of the requirements

for the degree of

Master of Science

in

Ocean Engineering

Melbourne, Florida

December, 2018

We the undersigned committee hereby approve the attached thesis, “The Corrosion

and Biofouling Characteristics of Sealed vs. Perforated Offshore Monopile

Interiors: Experiment Design Comparing Corrosion and Environment Inside Steel

Pipe,” by Monica M. Maher.

_________________________________________________

Geoffrey Swain, Ph.D.

Professor of Ocean Engineering and Marine Sciences

College of Engineering and Science

_________________________________________________

Stephen Wood, Ph.D., P.E.

Professor of Ocean Engineering and Marine Sciences


_________________________________________________

Troy Nguyen, Ph.D., P.E.

Associate Professor of Mechanical and Civil Engineering


_________________________________________________

Richard Aronson, Ph.D.

Professor and Head of Ocean Engineering and Marine Sciences


iii

Abstract

The Corrosion and Biofouling Characteristics of Sealed vs. Perforated Offshore Monopile

Interiors: Experiment Design Comparing Corrosion and Environment Inside Steel Pipe

Author: Monica M. Maher

Advisor: Geoffrey Swain, Ph.D.

This research addresses the need to improve on the existing methods for the corrosion

control of the monopile interiors used to support wind powered turbines by incorporating a

design that also enhances marine habitats and fisheries. Retrofitting monopile interiors

with cathodic protection has been attempted on existing windfarms to mitigate corrosion,

however, this can cause new problems including water acidification and hydrogen sulfide

formation. Such chemistry changes can lead to unique localized corrosion concerns.

This study investigated internal corrosion, chemistry and biofouling inside partially

submerged hollow steel pipes. Watertight pipes with stagnant water inside were compared

to pipes with holes that allow circulation with the surrounding seawater. By adding holes to

the structure walls, surrounding seawater consistently flushed the internal space. The goals

of opening up monopile walls were to improve corrosion control of internal surfaces and to

create an environment that enhances local ecosystems. With ambient seawater flushing,

cathodic protection design used for external surfaces can be applied to protect the interior

steel.

The field experiment presented here has demonstrated that a cathodically protected

perforated monopile structure created an environment with more favorable corrosion

mitigation and water chemistry compared to a sealed structure. Furthermore, the perforated

iv

cathodically protected pipe created a habitat for marine life and recruited a diverse

population of settled and mobile organisms. The development of healthy ecosystems

within a monopile structure would add to the benefits of constructing offshore windfarms.

v

Table of Contents

Table of Contents ..................................................................................................... v

List of Figures ........................................................................................................ vii

List of Tables ........................................................................................................ viii

Acknowledgement .................................................................................................. ix

Chapter 1 Introduction ............................................................................................ 1 Nomenclature ................................................................................................................... 1 Terms ................................................................................................................................ 1 Introduction ..................................................................................................................... 2 Industry Background ...................................................................................................... 3

Offshore Wind .............................................................................................................. 3 Corrosion Mitigation ..................................................................................................... 3 Stewardship ................................................................................................................... 4 Monopiles ..................................................................................................................... 4 Offshore Trial ................................................................................................................ 7

Chapter 2 Experiment Design ................................................................................. 8 Project Description .......................................................................................................... 8 Hypotheses ....................................................................................................................... 8 Materials .......................................................................................................................... 8

Component Design ...................................................................................................... 11 Corrosion Test Coupons.............................................................................................. 11 Electrical Connections ................................................................................................ 13

Methods .......................................................................................................................... 14 Deployment ................................................................................................................. 14 Weekly Measurements ................................................................................................ 16 End of Deployment ..................................................................................................... 19

Chapter 3 Results and Discussion ......................................................................... 20 Visual Observations ...................................................................................................... 20

Biofouling ................................................................................................................... 22 In Water Measurements ............................................................................................... 26

Potential Voltage ......................................................................................................... 26 Potentiodynamic Polarization ..................................................................................... 27 Water Chemistry ......................................................................................................... 31

Elemental Analysis ........................................................................................................ 35 Weight Loss .................................................................................................................... 39

Steel Coupons ............................................................................................................. 39 Zinc Anodes ................................................................................................................ 39

vi

Chapter 4 Conclusions ........................................................................................... 40 Conclusions .................................................................................................................... 40 Recommendations ......................................................................................................... 41

Future Work ................................................................................................................ 41

References ............................................................................................................... 43

Appendix A Polarization Data Processing ........................................................... 45

Appendix B ΔE/Δi Slopes from Ecorr ±25mV ..................................................... 46

Appendix C Tafel β Slopes .................................................................................... 47

Appendix D Calculations ....................................................................................... 48

Appendix E SEM-EDX Reports ........................................................................... 52

vii

List of Figures

Figure 1 — Offshore wind turbine support structure. ................................................ 4 Figure 2 — Monopile foundation. ............................................................................. 5

Figure 3 — Configuration of sealed pipes. ................................................................ 9 Figure 4 — Configuration of perforated pipes. ........................................................ 10 Figure 5 — Steel coupon.......................................................................................... 11 Figure 6 — Silver-silver chloride reference electrode. ............................................ 12

Figure 7 — 3-D printed spacer................................................................................. 12 Figure 8 — Wiring diagram. .................................................................................... 13 Figure 9 — Port Canaveral. ..................................................................................... 14 Figure 10 — Hanging pipes. .................................................................................... 15

Figure 11 — Drybox. ............................................................................................... 15

Figure 12 — YSIs. ................................................................................................... 16 Figure 13 — Potentiodynamic polarization equipment. .......................................... 17

Figure 14 — Corroded and fouled coupons. ............................................................ 20 Figure 15 — Sealed pipe interiors. .......................................................................... 21 Figure 16 — Perforated pipe interiors. .................................................................... 22

Figure 17 — Bottom cap of the unprotected perforated pipe. ................................. 23

Figure 18 — Bottom cap of the cathodically protected perforated pipe. ................. 23 Figure 19 — Mobile animals inside the unprotected perforated pipe. ..................... 24 Figure 20 — Mobile animals inside the cathodically protected pipe....................... 24

Figure 21 — Whole pipe rest potential. ................................................................... 26 Figure 22 — Tafel plots. .......................................................................................... 27

Figure 23 — Cathodic polarization resistance. ........................................................ 28 Figure 24 — Corrosion Rates over time. ................................................................. 30 Figure 25 — Dissolved oxygen................................................................................ 31

Figure 26 — pH. ...................................................................................................... 32 Figure 27 — Temperature. ....................................................................................... 34

Figure 28 — Salinity. ............................................................................................... 34

Figure 29 — Elemental composition sealed unprotected. ....................................... 37

Figure 30 — Elemental composition sealed with zinc............................................. 37 Figure 31 — Elemental composition perforated unprotected. ................................. 38 Figure 32 — Elemental composition perforated with zinc. ..................................... 38 Figure 33 — Max inertial force per length along a monopile during an extreme

wave event ........................................................................................................ 41

viii

List of Tables

Table 1 — Populations found during habitat evaluation ......................................... 25 Table 2 — Average weight loss (g) per 30cm2 coupon ........................................... 39

ix

Acknowledgement

Thank you to my colleagues at the Center for Corrosion and Biofouling Control (CCBC) at

Florida Tech for all of their support.

1

Chapter 1

Introduction

Nomenclature

Al = Aluminum

CP = Cathodic Protection

DNV = Det Norske Veritas

MIC = Microbiologically Influenced Corrosion

SEM-EDX = Scanning Electron Microscope – Energy Dispersive X-ray

SRB = Sulfate Reducing Bacteria

Zn = Zinc

Terms

Coupon: a small piece of steel flat bar used as a representative sample of steel pipe interior

surface

Monopile foundation: A single large hollow steel pile driven into the seabed to support an

entire offshore wind turbine structure

Perforated monopile: A monopile extending up through the water column with holes cut

through the submerged sides which allow exchange between internal and ambient seawater

Sealed monopile: A nearly airtight monopile extending up through the water column

containing stagnant seawater that was filled at the time of installation

2

Introduction

The offshore wind industry has experienced corrosion problems of the interior surfaces of

submerged steel monopile foundations. These structures are typically built from cold rolled

steel plate, up to 150mm wall thickness and welded together. Monopiles may be deployed

in water depths up to 30m, are typically 5-7.5m diameter, and 60+m in length. This

research investigated the feasibility of incorporating perforations in the steel monopile

walls that would allow the free flow of seawater into the interior. This would allow

conventional cathodic protection design for corrosion control and the creation of a habitat

for marine life.

Corrosion of the submerged internal surfaces of early windfarm monopiles was assumed to

be controlled by sealing the structures and preventing ingress of oxygen. Oxygen is the

main driver for corrosion reactions, so an enclosed submerged space was expected to have

limited initial corrosion which consumes the oxygen and then further corrosion during the

structure lifetime should not occur. The theory of passive corrosion mitigation in the

anaerobic space seemed reasonable and practical, however, field observations found

accelerated corrosion rates caused by imperfect seals allowing ingress of oxygen. This was

offset by retrofitting cathodic protection (CP), however, it was found that installing CP in

an enclosed space altered the water chemistry and created unique corrosion environments

[3].

This research investigated the concept that the addition of holes to the structure walls

would open the interior of the pile to flushing with ambient seawater. This would enable

corrosion control of the internal surfaces using conventional cathodic protection design and

also create a habitat for marine life that enhances local ecosystems.

3

Industry Background

Offshore Wind

Offshore wind energy is an emerging industry in the United States with several sites

already leased along the East Coast. Over the past three decades, offshore windfarm

structures have been designed using experience from offshore oil and gas platforms. More

recently, lessons learned from early European windfarms can be applied to new designs.

Bringing the benefits of offshore windfarms to fruition will require overcoming critical

challenges that include corrosion control and supporting stewardship of U.S. waters

[16][19].

Corrosion Mitigation

Corrosion is a major challenge and is especially prevalent in the marine industry. Globally,

corrosion problems and mitigation efforts cost trillions of dollars every year. Three main

strategies are used to control corrosion of steel structures in the marine environment:

protective coating, corrosion allowance, and cathodic protection. Coating the steel surface

provides a barrier from the chemically reactive seawater. Corrosion allowance consists of

increasing the thickness of structure members so that steel at the surface can be lost to

corrosion without jeopardizing the integrity of the structure.

Cathodic protection (CP) is a technique in which the electrochemical potential of the

structure metal is displaced to a value that prevents the corrosion reactions. This can be

achieved using sacrificial anodes that are more reactive on the galvanic series than the

structural metal. A current is supplied to the steel from anode materials like aluminum or

zinc, as they corrode and supply electrons. The anode materials are consumed over time

and are called galvanic or sacrificial anodes. A properly designed CP system will keep the

steel at an electric potential such that it is immune from corrosion for its design life.

Corrosion is successfully mitigated on many existing marine structures using CP.

However, unforeseen challenges have presented themselves when applying CP inside wind

turbine support tower monopile foundations.

4

Stewardship

Stewardship of coastal waters is required to ensure that fisheries, endangered species and

water quality are maintained and enhanced. The design and deployment of offshore

structures should therefore take into account the impact they will have on the marine

biology and ecosystem. This may be achieved by incorporating designs that improve the

habitat created for marine organisms.

Monopiles

Steel monopiles provide the most common foundation for offshore wind turbine support

towers. The foundation design under consideration is driven into the seabed, extends up

through the water column, and has an air gap at the top above water level (Figure 1). A

transition piece sits on top of the monopile extending the tower upwards towards the wind

turbine height. Even with the large cost of power generation turbines, the installation and

maintenance of foundations consumes a significant portion of windfarm lifetime costs.

Optimization is needed for corrosion control design [12].

Figure 1 — Offshore wind turbine support structure.

5

For many installations, the inside of the monopile foundations were designed to be air

tight. Seawater flooded the cavity upon installation, after which stagnant water and air

remained inside.

Det Norske Veritas (DNV) provides technical standards and design guidance. Offshore

Standard DNV-OS-J101, “Design of Offshore Wind Turbine Support Structures” includes

Section 11, which covers corrosion protection. In this standard, CP and coatings were

optional on submerged, sealed internal surfaces [5]. Active corrosion protection was

considered unnecessary inside the monopile structures because oxygen in the confined

environment would be consumed during some initial corrosion, and then corrosion would

stop once the water turned anaerobic.

Figure 2 — Monopile foundation.

6

The reality of the field, however, has proven inconsistent with this design theory.

Structures easily lose their airtight seal and develop small leaks around conduit

penetrations or cracked grout (Figure 2). These breaches allow oxygenated seawater to

enter and fuel corrosion on the internal walls of the structure. As the water is mostly

stagnant and sometimes replenished with small amounts of oxygen, unique corrosive

environments are created that increase the risks of both localized corrosion and

microbiologically influenced corrosion [1]. Several windfarms have experienced internal

corrosion at higher than anticipated rates [12].

In 2014, DNV-OS-J101 Section 11 was updated with a guidance note regarding

compartments exposed to air [2] but coating and CP are still optional for submerged

internal surfaces.

The DNV standard for general offshore steel structures also addressed internal zones. This

was written with ballast tanks in mind and requires coating on internal zones that are

exposed to seawater most of the time. Corrosion allowance is considered acceptable for

internal compartments with intermittent contact with seawater [6].

In 2016, a separate recommended practice was published, DNVGL-RP-0416 “Corrosion

protection for wind turbines.” In this document, CP is required on external surfaces of the

submerged zone, but CP and coating remain optional for internal submerged surfaces. A

guidance note advises that airtight seals are difficult to achieve in practice and that

anaerobic bacteria can cause corrosion even after the depletion of oxygen [7].

Based on field experience, new designs are more likely to consider coating the interior

before installation. Nevertheless, CP is still desirable as secondary corrosion protection

because damaged coatings can allow high rates of localized corrosion. Some installations

are still skipping interior coating and CP as an upfront cost saving measure since they are

not required by design standards.

7

Offshore Trial

Deploying CP inside the confined space can solve some issues but creates others. Alex

Delwiche published papers based on a case study in the North Sea in which sacrificial

aluminum anodes were retrofitted inside an existing monopile in an attempt to mitigate

corrosion. This offshore trial experienced air quality and water chemistry issues. These

included hydrogen gas and hydrogen sulfide (H2S) formation in the air gap at the top of the

monopile. Extreme water acidification also occurred during which pH dropped inside the

monopile from 8 to as low as 4.5. These water chemistry problems add to corrosion

concerns as they can cause unique types of corrosion and impede the effectiveness of

corrosion mitigation strategies. Acidic water can prevent the formation of calcareous scale

that would normally accumulate and protect the steel surface [4].

The observed water chemistry problems in that North Sea case study were specifically

attributed to the use of aluminum anodes. Metal ions in aqueous solution can cause

acidification based on their acid dissociation constant. From these values, Al3+ is expected

to produce a stronger acid than Zn2+ [2][3]. Aluminum anodes are preferred for most

applications in industry because they have a higher electrochemical capacity. They are also

lighter and easier to handle during installation.

In the North Sea trial, holes were eventually added to the monopile walls to allow limited

flushing with ambient seawater. Initial results showed promise for this investigational

solution with pH values moving toward neutral and decreasing levels of hydrogen sulfide

[4]. Opening monopiles with vent holes are also being considered elsewhere in industry

[12][14].

8

Chapter 2

Experiment Design

Project Description

Steel pipes were deployed to represent scaled down monopiles that support offshore wind

turbine support structures. Four different treatments were fashioned including two sealed

pipes and two perforated pipes. In each pair there was a freely corroding pipe and a

cathodically protected pipe with a zinc sacrificial anode. The sealed unprotected treatment

represents originally installed windfarms, sealed protected is similar to the case study

previously described [3], and perforated treatments were considered in this trial as a

potential improved design. Corrosion was evaluated by potential measurements,

polarization and weight loss. Biofouling and mobile creatures were observed to assess

habitat enrichment.

Hypotheses

A perforated structure will create an environment with more favorable corrosion mitigation

and water chemistry compared to the sealed structure.

A perforated structure will create a habitat for marine life and recruit a diverse population

of settled and mobile organisms.

Materials

One meter long, 15cm diameter steel pipes were used to represent monopiles sized

approximately 5m diameter and 40m length. Each pipe had a layer of local sediment at the

bottom to represent monopiles driven into the seabed. Seawater filled most of the pipe, and

an air gap was left at the top. Internal apparatuses were suspended in each pipe for

characterization of the steel surfaces and the enclosed seawater.

9

Figure 3 — Configuration of sealed pipes.

10

Figure 4 — Configuration of perforated pipes.

11

Component Design

A53 carbon steel schedule 10 pipes were sandblasted to clean steel and the pipe exteriors

were coated with epoxy and antifouling paint while the interiors were kept bare. Inside

each pipe were three sets of steel coupons that were removable to enable analysis. A silver-

silver chloride reference electrode was installed to measure the potential of the steel pipe

and coupons, and a counter electrode was placed near the middle coupon set for

polarization. One sacrificial (galvanic) zinc anode was placed in each of the cathodically

protected pipes. Zinc is recommended over aluminum for anaerobic conditions including

internal compartments [8]. Therefore, zinc anodes were installed in this experiment to see

if swapping aluminum for zinc is itself a viable solution. The perforated pipe walls each

had six holes sized 5cm diameter. The sealed pipes were each fitted with an underwater

PVC poke hole for probe measurements.

Figure 5 — Steel coupon.

Corrosion Test Coupons

Nine rectangular carbon steel coupons were suspended in each pipe. Each steel coupon was

cut from sandblasted A36 carbon steel 3mm thick flat bar. Marine grade wire was

connected to the top of each coupon using a ring terminal and stainless steel screw. The

screw and area around the connection were covered in clear epoxy leaving 30cm2 exposed

12

surface area of steel (Figure 5). The opposite end of each wire had a spade terminal,

accommodating easy connection to busbars. The busbars were made from a strip of copper

attached to rectangular PVC. Regularly spaced stainless steel screws penetrating the copper

and PVC strips were loosened and tightened to add and change component connections.

The counter electrodes were cut from a ribbon of mixed metal oxide mesh. They were

wired via an epoxy covered stainless screw in the same manner as the coupons. The

reference electrodes were fabricated from 8cm of medical grade silver wire and silver-

chloride powder. The wire was curled into a spoon shape at one end and dipped in molten

silver-chloride. The straight end of the silver wire was soldered to marine grade wire. The

connection was shrink-wrapped and stuck through the cap of a small hot water PVC tube

(Figure 6). A 10cm length of said tube was perforated with several holes to house the

silver-silver chloride electrode along with wool stuffing. A bottom end cap was added for

integrity.

Figure 6 — Silver-silver chloride reference electrode.

Figure 7 — 3-D printed spacer.

13

Plastic spacers were 3-D printed for each coupon set (Figure 7). These had a circular core

to which the coupons were strapped. They also had three protruding arms to act as bumpers

and ensure the steel coupons did not touch the pipe walls. This way the electrical

connections were completely controlled at the copper busbars.

The PVC poke holes were fitted with screw caps that were kept closed during the week. A

slit rubber gasket was added to mitigate water ingress while the cap was removed for probe

measurements.

Electrical Connections

A copper busbar for each pipe kept all of the system components electrically connected.

Each coupon was wired in series with the pipe and zinc anode through the copper busbar.

Smaller “sub” busbars were fabricated that connect the middle coupons in each pipe. This

smaller bar could easily be detached and reconnected to the whole pipe system busbar. The

middle coupon set, reference electrode, and counter electrode were wired to banana plugs

for potential and polarization measurements (Figure 8). The copper busbars were housed in

a drybox with all of the wiring fed through a downward facing PVC elbow on the front of

the drybox (Figure 11).

Figure 8 — Wiring diagram.

14

Methods

Deployment

The experiment was deployed from the Center for Corrosion and Biofouling Control

floating dock at Cape Marina, Port Canaveral, a manmade seaport on Florida’s East Coast

(Figure 9). Deployment was from August 3rd to October 1st, 2018.

Figure 9 — Port Canaveral.

Deployment in natural seawater was important to obtain realistic results for this

experiment. Natural seawater is more corrosive than synthetic because of microbiologically

influenced corrosion (MIC) in which dead and alive organisms cause corrosion.

Warmer waters have lower oxygen content than cold, so corrosion rates tend to be lower in

Floridian waters than in the North Sea. However, two months deployment of carbon steel

was estimated to be sufficient time to observe enough corrosion to distinguish the four

different treatments. Different biofouling species were also expected for this nearshore,

subtropical zone compared to existing European windfarms [17]. However, general

patterns of the amount and community structure of biofouling were assumed to provide

insight that is applicable to forthcoming Mid-Atlantic American windfarms as well as

international ones.

15

The pipes were hung from the floating dock so the water level inside and around the pipes

did not oscillate with tides. However, the water level did fluctuate on the order of a few

centimeters because of small waves and shifting weight distribution on the floating dock.

The copper busbars were housed in a drybox.

Figure 10 — Hanging pipes.

Figure 11 — Drybox.

16

Weekly Measurements

Weekly measurements were taken related to steel corrosion and internal water chemistry.

Measurements were chosen based on monitoring practices of existing windfarms and

corrosion industry techniques. These include visual evaluation, pH, dissolved oxygen,

electric potentials, polarization resistance and weight loss, see: [9][10][13][14].

YSI probes were used to measure pH and dissolved oxygen (DO) levels. A salinity probe

was also used to supplement these measurements. Temperature and pressure were recorded

from the DO probe.

Each of the sealed pipes was fitted with a PVC poke hole (Figure 3). The poke hole

provided access for probes to take measurements in the internal water column. The

perforated pipes were probed through one of the cut holes at a similar height to the poke

holes. Each week, probe measurements were taken in all four pipes, usually within one

hour of high tide. Therefore, all four pipes were measured at approximately the same time,

though the time of day for measurements varied from week to week.

Figure 12 — YSIs.

17

A potentiostat, ramp generator, and data logger were used to for potentiodynamic

polarization measurements on one set of steel coupons in each pipe (Figure 13).

Multimeters were used for potential measurements and as quality control during

polarization.

Figure 13 — Potentiodynamic polarization equipment.

Data Logger

Monitoring

Data Logger

Potentiostat

Ramp

Generator

Multimeters

Busbar

connected

to coupon

set

(working

electrode)

18

The corrosion characteristics of the steel were measured using the following procedures.

First, the potential of the connected pipe, coupons, and zinc anode system was recorded

referenced to a silver-silver chloride electrode. Then the middle set of three coupons were

disconnected from the system and given time to normalize once separated from the

structure (Figure 8). The rest potential of the isolated coupon set was measured. The

middle coupon set was polarized cathodically and then returned to its rest potential. The

applied potential and current response were recorded. After waiting an hour to normalize

again, the middle coupon set was polarized anodically and again returned to its rest

potential while recording applied potential, E, and current response, I.

It is important to note that these electrochemical techniques are best suited to provide

insight on uniform, or general, corrosion rates. The applied methods did not have the

capability to detect unique corrosion phenomena that may occur in localized areas

throughout the steel surface [13].

19

End of Deployment

Biodiversity was considered in this experiment to determine the success and health of the

pipe habitats. Biology observations were made on the last day of experiment deployment

before and after removing the pipes from the seawater. First, the top caps were removed

along with the internal apparatus. These were observed and photographed on the floating

dock. A sealed canister was used to take a water sample. Video was recorded panning the

pipe interiors. A framed net made of plankton mesh was placed around each of the

perforated pipes to catch the mobile creatures that were inside at the time of lifting them

out of the water. These animals were placed in a white tray for population count, species

identification and photographing. When removing the bottom caps, an additional check

was made for mobile creatures and a mud sample was taken.

The steel coupons and pipe interior walls were visually inspected for biofouling. The

coupon sets were also weighed to get a sense of the amount of biofouling accumulated.

The steel coupons were cleaned of biofouling and inspected further for corrosion and

precipitate deposits. A 70mm square sample was cut from each coupon set for use in SEM-

EDX analysis. The other two coupons from each set were thoroughly cleaned using a brush

and an acid wash to remove corrosion products and leave only bare steel. Those coupons

were then weighed and compared to their original pre-deployment weight to determine the

weight of steel loss.

The zinc anodes were inspected and weighed for comparison between the sealed and

perforated treatments.

20

Chapter 3

Results and Discussion

Visual Observations

At the end of the experiment deployment, the steel surfaces were visually inspected for

corrosion and biofouling. The coupons in the sealed, freely corroding treatment had black

and orange corrosion products. This treatment represents the originally installed

unprotected windfarm structure interiors. The sealed with CP treatment had thick white

deposits on the submerged surfaces. The perforated, freely corroding treatment had

biofouling and corrosion products. The perforated, cathodically protected treatment had no

corrosion and the surfaces had become colonized by a healthy biofouling community.

Figure 14 — Corroded and fouled coupons.

Sealed unprotected Sealed with zinc Perforated unprotected Perforated with zinc

21

Immediately upon opening the top cap of the sealed cathodically protected treatment, a

white gas was seen, and a rotten egg odor was noted which was likely H2S smell. However,

air samples were not taken for analysis.

The splash zones of the sealed pipes exhibited raised corrosion products (Figure 15). The

dramatic tubercles protruding from the surface of the cathodically protected pipe were

especially fascinating. Corrosion tubercles are potentially, though not exclusively,

indicative of non-uniform corrosion reactions including MIC [11]. Elemental analysis

included a search for sulfur as this could further imply MIC caused by sulfate reducing

bacteria (SRB). Localized pitting was not observed on any coupons. This would be more

likely to occur during a longer deployment. The pipe interior walls were not cleaned for

steel surface inspection, so it is unknown if pitting occurred there.

Figure 15 — Sealed pipe interiors.

Sealed unprotected Sealed with zinc

22

Biofouling

The sealed pipes had no macrofouling and were therefore biologically unproductive. The

interior surfaces of both the perforated pipes were covered in abundant biofouling (Figure

16).

Figure 16 — Perforated pipe interiors.

The perforated pipes had diverse biofouling with similar species observed in each

including: encrusting bryozoan, arborescent bryozoan, sauerkraut bryozoan, tubeworms,

tunicates, invasive Balanus amphitrite barnacles and native Balanus eburneus barnacles.

One oyster was observed in the cathodically protected pipe. There were, however,

significant differences in the community structure. The marine growth inside the freely

corroding pipe was impoverished due to sloughing caused by corrosion of the steel. This

prevented the development of a stable community and also led to the accumulation of dead

organisms at the base of the pipe that created anoxic muds (Figure 17). The marine growth

in the cathodically protected pipe developed into a more diverse and stable community

with nearly 100% biofouling cover and a higher population count of macro mobile animals

than in the freely corroding pipe.

Perforated unprotected Perforated with zinc

23

Figure 17 — Bottom cap of the unprotected perforated pipe.

Figure 18 — Bottom cap of the cathodically protected perforated pipe.

In industry, biofouling is sometimes credited with providing corrosion protection as the

fouling organisms shield the steel surface from corrosive seawater. However, the

underlying steel must have enough integrity to support the accumulated fouling.

Furthermore, layers of decaying marine growth can cause local anoxic conditions and

support SRB [12], thus causing MIC and air quality concerns.

24

Figure 19 — Mobile animals inside the unprotected perforated pipe.

Fish and crustaceans inhabited both perforated pipes. The unprotected pipe had a

schoolmaster snapper, five blue crabs, and amphipods (Figure 19). A larger population of

mobile creatures was inside the cathodically protected pipe. The inhabitants included a

small schoolmaster snapper, two frillfin gobies, two blennies, two blue crabs, three other

Callinectes crabs, 18 daggerblade grass shrimp, two pink shrimp, and amphipods (Figure

20). Six of the shrimps were gravid.

Figure 20 — Mobile animals inside the cathodically protected pipe.

25

Table 1 — Populations found during habitat evaluation

Perforated Unprotected

Perforated with Zn

Mo

bile

Cre

atu

res

Schoolmaster snapper 1 1

Frillfin goby 2

Blenny 2

Blue crabs 5 2

Other Calinectes crabs 3

Daggerblade shrimp 18

Pink shrimp 2

Amphipods X X

Bio

fou

ling

Encrusting bryozoan X X

Arborescent bryozoan X X

Sauerkraut bryozoan X X

Tubeworms X X

Tunicates X X

Barnacles (native & invasive) X X

Oyster 1

26

In Water Measurements

Potential Voltage

The rest potential (reference silver-silver chloride) of each pipe system was measured

weekly. The four treatments quickly developed different potentials as the zinc anodes

polarized the pipe systems. A potential more negative than -800mV is required to keep the

steel immune from corrosion in seawater. Adequate protection was achieved and

maintained for both the sealed and perforated treatments with zinc sacrificial anodes.

Figure 21 — Whole pipe rest potential.

-1100

-1050

-1000

-950

-900

-850

-800

-750

-700

-650

-600

3-Aug 10-Aug 17-Aug 24-Aug 31-Aug 7-Sep 14-Sep 21-Sep 28-Sep

Pip

e R

est

Po

ten

tial

mV

2018

Whole Pipe System Rest Potential

Sealed Freely Corroding Sealed Zn Perforated Freely Corroding Perforated Zn

27

Potentiodynamic Polarization

Every week, the middle set of coupons in each pipe was polarized cathodically and

anodically 100mV from the rest potential of the isolated coupon set. The applied voltage

was plotted against the measured current response to produce a Tafel plot.

Figure 22 — Tafel plots.

-1100

-1000

-900

-800

-700

-600

-500

0.1 1 10

Ap

pli

ed V

olt

age

(mV

)

Current Response (mA)

Tafel PlotsSept 14

Sealed Unprotected Anodic Sealed Zn Anodic Perforated Unprotected Anodic Perforated Zn Anodic

Sealed Unprotected Cathodic Sealed Zn Cathodic Perforated Unprotected Cathodic Perforated Zn Cathodic

28

Looking at only the first 25mV of cathodic polarization plotted on a linear scale, the slopes

ΔE/Δi, were recorded each week and compared. The steeper the slope, the higher the

resistance to cathodic polarization. Resistance is a function of oxygen availability and

cathodic chalk or corrosion layers formed. A high resistance indicates a lower rate of

corrosion. The resistances in the sealed environments varied dramatically during the first

month as the water chemistry was changing. In the second month of deployment, the four

different treatments exhibited different resistances (Figure 23).

Figure 23 — Cathodic polarization resistance.

The sealed cathodically protected treatment had the highest resistance and the perforated

freely corroding treatment had the lowest resistance. These rankings were expected as

cathodic polarization resistance increases with the reduction of oxygen availability and the

formation of cathodic chalks. Chalk deposits act as a coating and increase resistance. The

average cathodic resistance of the sealed freely corroding and perforated cathodically

-350

-300

-250

-200

-150

-100

-50

0


Cat

ho

dic

Slo

pe

∆E

/∆i

2018

25mV Cathodic Slope ∆E/∆i


29

protected steel ranked in the middle. Considering statistic variance, these second and third

rankings could not be established with confidence.

It should be noted, however, that the cathodic polarization resistance was measured on

steel coupons that had been disconnected from the whole pipe systems. During the week,

the coupons were connected with their respective whole pipe system including the zinc

anode for the cathodically protected treatments. Therefore, these cathodically protected

coupons were not corroding at the rates suggested by the polarization plots. The

disconnected measurements provide an indication of cathodic chalk formation on the steel

surface and of the corrosivity of the environment inside the pipes.

Equations 1, 2, and 3 are used in lab studies to quantify corrosion rates [15][18]. The

corrosion current, iCORR, is related to the slopes of the Tafel plot curves.

𝐄 − 𝐄𝐂𝐎𝐑𝐑 = 𝛃𝐥𝐨𝐠𝐢

𝐢𝐂𝐎𝐑𝐑 (1)

𝐢𝐂𝐎𝐑𝐑 = 𝛃𝐀𝛃𝐂

𝟐.𝟑(𝛃𝐀+𝛃𝐂)

∆𝐈

∆𝐄 (2)

Where E is the applied potential and i is the current response. βA is the anodic Tafel

constant and βC is the cathodic Tafel constant. These values are the slopes of the linear

portions of the Tafel plot curves with current plotted on logarithmic scale. These slopes

intersect each other at the point (ECORR, iCORR) where iCORR is the corrosion current.

In order to get the linear slopes of the Tafel plot, it is often necessary to polarize 200mV or

300mV cathodically and anodically [15]. However, polarizing this much will corrode the

steel coupons and make them unfit for continued use as specimen. Curves were plotted

after polarizing 100mV cathodically and anodically, so the β slopes may be inaccurate

(Error! Reference source not found.). For this field experiment, it was decided to p

reserve the integrity of the coupons by limiting polarization to ±100mV.

30

On the limited Tafel plots, the log slopes were extrapolated to estimate the cathodic and

anodic intersection point (ECORR, iCORR). The corrosion rate is calculated using iCORR, the

equivalent weight (E.W.) of the steel (g), and the density (ρ) of the steel (g/cm2).

𝐂𝐨𝐫𝐫𝐨𝐬𝐢𝐨𝐧 𝐑𝐚𝐭𝐞 = 𝟎.𝟏𝟑𝐢𝐂𝐎𝐑𝐑(𝐄.𝐖.)

𝛒 (3)

The corrosion rate is estimated in mils per year, or thousandths of an inch of steel thickness

lost per year. As expected, the sealed cathodically protected treatment had the lowest rate

of corrosion and the perforated unprotected treatment had the highest rate of corrosion. The

estimated corrosion rates were similar for the sealed unprotected treatment and the

perforated treatment protected with zinc. Again, these corrosion rates were estimated using

calculations based on the disconnected coupons. So actual corrosion rates were lower in

the cathodically protected pipes during the week when the samples were connected in

series with the zinc sacrificial anode.

Figure 24 — Corrosion Rates over time.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0


Co

rro

sio

n R

ate

(m

py

)

2018

Corrosion Rate (mpy)


31

Water Chemistry

Oxygen levels of seawater at the port fluctuate on a daily cycle related to biological

activity. Each week, measurements were taken at different times of day.

Figure 25 — Dissolved oxygen.

DO was quickly reduced in the sealed pipes, in line with the theory of oxygen getting

consumed during initial corrosion. It was expected that the water would become anoxic

rather than completely anaerobic. Some oxygenated seawater ingress occurred each time

the perforated pipes were probed. The poke hole was designed to mitigate, though not

eliminate, the entrance of ambient seawater. Therefore, the pipes were not completely

sealed, especially as the rubber gaskets wore down overtime. DO in the perforated pipes

matched that of ambient seawater for the first month. The decrease in the second month

could be explained by the growing biological community consuming oxygen. As

0

1

2

3

4

5

6

7

8

9


DO

mg/

L

2018

Dissolved Oxygen

Ambient Sealed Freely Corroding Sealed Zn Perforated Freely Corroding Perforated Zn

32

macrofouling developed on the pipe interiors oxygen was consistently consumed, and

ambient seawater was exchanged through the perforations.

Ambient seawater stayed neutral near pH of 8.2. In the sealed unprotected treatment, pH

increased after oxygen was reduced. Acidification occurred in the sealed pipe with zinc,

similar to the real-world North Sea field trial with aluminum anodes. Acidification is a

corrosion concern as it can prevent protective scales from forming on the structure surface

and favors the anodic reaction. The perforated pipes maintained pH similar to neutral

ambient seawater (Figure 26).

Figure 26 — pH.

6.5

7

7.5

8

8.5

9

9.5


pH

2018

pH


33

In the sealed unprotected pipe, oxygen was consumed during initial corrosion reactions.

The electrochemical reactions that follow may be akin to those of crevice corrosion. In

crevice corrosion, the electrolyte in the shielded crevice becomes anaerobic due to lack of

oxygen ingress and then becomes acidic. The dissolution of iron, an anodic reaction, takes

place inside the crevice (Equation 4). These electrons flow away from the crevice, through

the steel to the unshielded steel surface where they react with oxygen and water in a

cathodic reaction and form hydroxyl ions, OH-, which are alkaline (Equation 5). With the

loss of electrons in the crevice, the trapped electrolyte becomes positively charged and then

attracts chlorine ions, Cl-, from the seawater. Hydrogen chloride, HCl, is formed (Equation

6) and acidification occurs locally in the crevice.

Fe Fe+2+2e- Anodic reaction (4)

O2+2H2O+4e- 4OH- Cathodic reaction (5)

FeCl2+2H2O ↔ Fe(OH)2+2HCl (6)

However, after iron oxide Fe(OH)2 corrosion products are formed, these could be reduced

in reactions that neutralize pH. Furthermore, the formation of hydroxyl ions dominates and

is a more efficient process than the acidifying formation of hydrogen chloride [3]. If both

reactions take place in a closed compartment, they should alkalize the water to a pH above

neutral. After a couple weeks of deployment, the water inside the sealed freely corroding

pipe turned alkaline and pH measured around 8.9 (Figure 26).

When sacrificial CP is applied in an enclosed seawater environment, then hydrolysis of

anode corrosion products consumes oxygen and causes acidification [3]. Aluminum in

aqueous solution may form complex ions that cause acidification, reducing the pH to as

low as 4. Similarly, zinc in aqueous solution may push the electrolyte pH down to 6.8 [2].

This theoretically calculated equilibrium pH is consistent with the measured pH values

near 6.8 in the sealed cathodically protected treatment (Figure 26).

34

Temperature and salinity were measured to supplement the YSI measurements of DO and

pH. As expected, the temperatures in each pipe were near that of ambient (Figure 27). It

was expected that the salinities of the perforated pipes would be the same as ambient,

however there were some differences in the measurements (Figure 28). No obvious trend

was observed for the perforated pipes. The salinities measured in each of the sealed pipes

were similar to each other throughout deployment.

Figure 27 — Temperature.

Figure 28 — Salinity.

24

26

28

30

32


Tem

per

atu

re (

℃)

2018

Temperature


29

31

33

35

37


Sali

nit

y (

‰)

2018

Salinity


35

Elemental Analysis

The elemental composition of corrosion products and steel samples was investigated using

Florida Tech’s scanning electron microscope (SEM) and energy dispersive X-ray (EDX).

Samples were analyzed from the middle coupons sets, the bottom coupon sets, and

corrosion product scrapings from the pipe interior walls at the splashzone.

The occurrence of sulfur was noted as this may indicate the presence of SRB. After oxygen

is depleted, bacteria will reduce alternative electron acceptors such as nitrate and then

sulfate. SRB create H2S, hydrogen sulfide (Equation 7). Sulfur was detected at a weight

percent greater than 1% in both of the sealed pipes. A trace amount of sulfur was detected

in the perforated with zinc treatment.

SO4-2+H+

H2S+H2O+e- (7)

The sealed freely corroding submerged surface had iron oxide corrosion products (Figure

29). Similarly, the perforated freely corroding submerged surface was also dominated by

iron oxide corrosion products (Figure 31).

The sealed cathodically protected submerged surface was covered with zinc carbonate

(ZnCO3) chalking (Figure 30). The white chalking was so thick that iron from the

underlying steel was undetected. Significant amounts of zinc covered the surfaces even

though the zinc anode was minimally consumed in the sealed treatment. Calcium was not

detected on the middle coupon sample and only a small quantity was detected on the

bottom coupon sample, indicating a lack of calcareous scale. Typically, when CP is

applied, pH increases immediately next to the steel surface and this attracts calcareous

deposits. Acidification of the internal water prevented calcareous coating of the steel.

One theory suggested that hydroxyl ions may be consumed in calcareous scale formation

and therefore be unavailable for pH increase [3]. This contradicts the apparent lack of

calcareous scale development in the sealed treatment with zinc. However, the extensive

36

chalking made up of zinc carbonate would similarly consume hydroxyl ions, thus leaving

them unavailable to neutralize the acidic solution.

In contrast, zinc was not detected on the submerged surfaces of the perforated pipe

protected with a zinc anode (Figure 32). Calcium carbonate (CaCO3) appears to make up

the thin calcareous scale present, and this is typical of traditional cathodic protection

reactions. This type of insoluble scale acts as a barrier that helps protect the steel substrate

from further corrosion.

37

Figure 29 — Elemental composition sealed unprotected.

Figure 30 — Elemental composition sealed with zinc.

38

Figure 31 — Elemental composition perforated unprotected.

Figure 32 — Elemental composition perforated with zinc.

39

Weight Loss

Steel Coupons

Uncertainties in the weight measurements and the limited number of samples did not

support statistically significant quantitative weight loss declarations for each coupon.

However, qualitatively, there was a clear trend indicating the least steel loss was in the

sealed protected treatment and the most steel loss in the perforated unprotected treatment

(Table 2). This is consistent with the polarization analysis and also consistent with

theoretical expectations.

Table 2 — Average weight loss (g) per 30cm2 coupon

Sealed with Zn

Perforated with Zn

Sealed Unprotected


Average weight loss 0.66 0.97 0.99 2.53

Zinc Anodes

There was a notable difference in the consumption of the zinc sacrificial anodes during the

two-month deployment. The anode in the sealed pipe had minimal weight loss while the

anode in the perforate pipe lost about 20% of its original weight.

Cathodic protection design considers an initial current density required for keeping steel

immune from corrosion as well as a lower mean current density required over the lifetime

of the structure. Formation of calcareous scale is one a reason why the mean design current

density is about half of the initial design current density. Even though the anode

consumption in the perforated pipe was significant in this two-month trial, its consumption

rate over a longer deployment would decrease while it only had to maintain a mean current

density.

40

Chapter 4

Conclusions

Conclusions

The results from this research confirmed both hypotheses:

Interior walls of perforated monopiles can more easily and predictably be protected

from corrosion using cathodic protection as compared to sealed monopiles.

Cathodically protected perforated monopiles will enhance fisheries and ecosystems

by providing a sheltered space within which marine organisms prosper.

It was possible to protect the perforated pipe with a conventional sacrificial cathodic

protection system. CP on interior walls can therefore be designed similarly to external

surfaces. Consistent flushing of ambient seawater prevents water chemistry problems and

unique, localized corrosion concerns of submerged, sealed compartments. Protective

calcareous scales form in line with traditional CP design. Effective and predictable CP will

extend the service life of structure foundations and neutral seawater chemistry will ensure

safer confined work spaces for maintenance activities. Perforations should be incorporated

into the design of offshore wind turbine support structures to capture these benefits.

Additionally, it was found that the cathodically protected perforated monopile created a

habitat for benthic and mobile marine organisms. Similar to an artificial reef, perforated

monopiles could enhance regional ecosystems and potentially add economic benefit for the

seafood and sport fishing industries. Such economic benefits can help win citizen support

for offshore windfarms.

This experiment demonstrated that solution strategies for corrosion problems on offshore

windfarms can be studied through cost-effective, scaled-down, near-shore trials.

Specifically, this low-cost experiment on the Florida coast used approximately 1:30 scale

pipes to successfully replicate chemistry phenomena observed in functioning North Sea

41

windfarms. Additional simple experimental designs could yield further results relevant to

industry applications.

Recommendations

The next step in making this innovation commercially available is to transfer this

knowledge to the industry and to strategically design perforations to be incorporated into

monopile configurations. Challenges include modeling hydrodynamic changes in wave and

current loading and to modify the monopile dimensions and wall thickness accordingly.

Designing with biofouling in mind could also be considered as it relates to logistical

considerations of long-term maintenance and structure management.

Future Work

The placement of perforations and their effect on hydrodynamic loading should be studied.

For example, considering a typical plot of wave loading on a solid offshore monopile

(Figure 33), the most extreme wave force is found at the water surface and force is minimal

half way down the structure to the seafloor. The arrangement and shape of perforations will

alter the loading due to waves and current.

Figure 33 — Max inertial force per length along a monopile during an extreme wave event

42

The accumulation of biofouling on the exterior surface will also increase the effective

diameter and area, and change the inertial and drag coefficients. Future research should

study the impact perforations and biofouling have on the loading of monopiles.

This experiment only focused on the corrosion of steel in the water column. Supplemental

studies should concentrate on the splashzone or the mudzone of the monopile surface. MIC

is particularly of concern in these areas.

43

References

[1] A.R. Black, T. Mathiesen, L.R. Hilbert. (2015). “Corrosion protection of offshore

wind foundations,” NACE Corrosion-2015, paper no. C-2015 5896.

[2] S.T. Briskeby, L. Borvik, S.M. Hesjevik. (2015). “Cathodic protection in closed

compartments – pH effect and performance of anode materials,” NACE Corrosion-

2015, paper no 5657.

[3] A. Delwiche, P. Lydon, I. Tavares. (2017). “Concerns over utilizing aluminum

alloy anodes in sealed environments,” NACE Corrosion-2017, paper no C-2017

8956.

[4] A. Delwiche, I. Tavares. (2017). “Retrofit strategy using aluminum anodes for the

internal sections of windturbine monopiles,” NACE Corrosion-2017, paper no C-

2017 8955

[5] DNV-OS-J101, (2014). “Design of offshore wind turbine structures,” Hovik,

Norway: DNV, May 2014.

[6] DNVGL-OS-C101, (2015). “Design of offshore steel structures, general-LRFD

method” Hovik, Norway: DNV, July 2015.

[7] DNVGL-RP-0416, (2016). “Corrosion protection for wind turbines,” Hovik,

Norway: DNV: March 2016

[8] DNVGL—RP-B401, (2017). “Cathodic protection design,” Hovik, Norway: DNV,

June 2017.

[9] L.R. Hilbert, A.R. Black, F. Andersen, T. Mathiesen. (2011). “Inspection and

monitoring of corrosion inside monopile foundations of offshore wind turbines,”

EUROCORR 2011, paper no 4730.

[10] B.B. Jensen, (2015) “Corrosion protection of offshore windfarms, protecting

internal side s of foundations,” NACE Corrosion-2015, paper no. C-2015 5762.

[11] B.J. Little, R.I. Ray, Lee, J.S. (2010). “Tubercles and localized corrosion on carbon

steel,” Corrosion Management. Vol 98, p 12-15.

[12] T.N. Lomholt, T. Mathiesen, S. Egelund. D.B. Bangsgaard. (2018). “Unification of

corrosion protection for offshore windfarms – collaboration in partnerships,”

NACE Corrosion-2018, paper no C-2018-11170.

44

[13] F. Mansfeld, B. Little. (1992). “Electrochemical techniques applied to studies of

microbiologically influenced corrosion (MIC),” Trends in Electrochemistry, 1.

[14] T. Mathiesen, A. Black, F. Gronvold. (2016). “Monitoring and inspection options

for evaluating corrosion in offshore wind foundations,” NACE Corrosion-2016,

paper no C-2016 7702.

[15] Princeton Applied Research. “Potentiodynamic polarization measurements,”

application note CORR-1

[16] Schwarz, Heimiller, Haymes, Musial. (2010). “Assessment of Offshore Wind

Energy Resources for the United States,” Technical Report NREL/TP-500-45889.

Golden, CO: National Renewable Energy Laboratory.

www.nrel.gov/docs/fy10osti/45889.pdf.

[17] G. Swain. (2017). “A guide to developing a biofouling management plan,” Marine

Technology Society Journal. Vol 51, Issue 2.

[18] D. Townley, et al. (1998). “Design of galvanic anode cathodic protection systems

for offshore structures,” NACE International, paper no 24196.

[19] US Dept of Energy. (2016). “National Offshore Wind Strategy,”

45

Appendix A

Polarization Data Processing

As the coupons were polarized step-wise, the raw data had a range of current response for

each applied potential. The minimum current response was extracted for Tafel plots.

One example is shown here:

-760

-740

-720

-700

-680

-660

-640

-620

0.001 0.01 0.1 1 10Ap

pli

ed P

ote

nti

al (

mV

)


Perforated Zn Anodic Sept 14

-880

-860

-840

-820

-800

-780

-760

-740

0.1 1 10Ap

pli

ed P

ote

nti

al (

mV

)


Perforated Zn Cathodic Sept 14

46

Appendix B

ΔE/Δi Slopes from Ecorr ±25mV

y = 37.603x - 652.07

y = 48.72x - 885.63

y = 5.6011x - 720.31y = 36.305x - 743.59

y = -37.811x - 638.6

y = -56.324x - 869.58

y = -10.625x - 712.75

y = -39.521x - 740.92

-950

-900

-850

-800

-750

-700

-650

-600

-550

-500

0 1 2 3 4 5

Ap

pli

ed V

olt

age

(mV

)


ΔE/Δi Slopes from Ecorr ±25mVSept 14



47

Appendix C

Tafel β Slopes

y = 37.548ln(x) - 608.69

y = 87.308ln(x) - 801.73

y = 31.035ln(x) - 748.55

y = 57.686ln(x) - 708.01

y = -53.57ln(x) - 677.36

y = -107.4ln(x) - 963.57

y = -90.51ln(x) - 663.02y = -293.3ln(x) - 826.59

-1100

-1000

-900

-800

-700

-600

-500

0.1 1 10

Ap

pli

ed V

olt

age

(mV

)


Tafel β SlopeSept 14



48

Appendix D

Calculations

Sizing the zinc anodes

steel area 0.4 m2

polarize to potential -0.95 to -1.05 V initial 0.15 A/m2

final 0.1 A/m2

mean 0.07 A/m2

time 6 months 4380 hrs zinc z 780 Ah/kg

closed circuit potential -1 V

U 0.85

seawater resistivity 0.2 ohm*m

I=deltaV/R

deltaV -0.25 V

initial current required 0.06 A

zinc density 7.13 g/cm3

weight 0.39638009 kg for 6 months

need zinc anode volume 3.39246876 in3 55.59 cm3

Martyr zinc anode half 3.87 in3 63.37 cm3 good

rho 0.258

side 1 L 2.5 in 6.35 cm

side 2 2.25 in 5.715 cm

side 3 11/16 in 1.746 cm

c 3 3/32 in 7.858 cm

r 33/67 in 1.251 cm

49

Corrosion rate calculations

25mV Cathodic Slope ∆E/∆i

Sealed Unprotected

Sealed with Zn


Perforated with Zn

8-Aug -286 -39 -15 -16

13-Aug -111 -141 -13 -21

17-Aug -17 -304 -17 -19

24-Aug -14 -196 -25 -16

31-Aug -17 -126 -27 -18

7-Sep -31 -61 -23 -24

14-Sep -38 -56 -11 -40

21-Sep -33 -40 -6 -22

28-Sep -31 -57 -4 -34

25mV Anodic Slope ∆E/∆i

Sealed Unprotected

Sealed with Zn


Perforated with Zn

8-Aug 313 58 5 7

13-Aug 26 4 8 16

17-Aug 8 253 16 16

24-Aug 9 158 14 18

31-Aug 34 103 11 23

7-Sep 15 64 8 24

14-Sep 38 49 6 36

21-Sep 38 55 3 24

28-Sep 29 72 3 47

50

βC (log)

Sealed Unprotected

Sealed with Zn


Perforated with Zn

8-Aug -46.9 -57.3 -156.8 -107.7

13-Aug 0.0 -32.6 -92.5 -96.4

17-Aug -62.5 -28.7 -108.6 -109.4

24-Aug -40.4 -30.8 -69.1 -65.1

31-Aug -35.6 -33.4 -95.5 -68.6

7-Sep -23.0 -33.4 -66.0 -85.1

14-Sep -23.0 -46.5 -39.5 -127.2

21-Sep -38.2 -52.1 -16.5 -103.8

28-Sep -29.1 -56.0 -43.0 -118.1

βA (log)

Sealed Unprotected

Sealed with Zn


Perforated with Zn

8-Aug 7.8 60.4 26.1 25.6

13-Aug 5.2 19.5 18.2 20.4

17-Aug 9.6 21.7 20.8 23.0

24-Aug 15.6 23.0 18.7 19.5

31-Aug 36.9 30.0 15.2 22.1

7-Sep 16.1 36.5 13.5 21.7

14-Sep 16.5 37.8 13.5 25.2

21-Sep 23.9 37.3 33.0 24.8

28-Sep 13.9 32.6 20.8 22.1

51

icorr from Tafel plots

Sealed Unprotected

Sealed with Zn


Perforated with Zn

8-Aug 0.15 0.8 4.5 3

13-Aug 0.3 2 1

17-Aug 1.2 0.07 1.2 1.1

24-Aug 1.5 0.1 1.2 1.1

31-Aug 0.8 0.1 1.2 0.8

7-Sep 0.9 0.35 1 0.8

14-Sep 0.5 0.4 2 0.7

21-Sep 0.54 0.45 4 0.75

28-Sep 0.5 0.4 7 0.6

Corrosion Rate (mpy)

Sealed Unprotected

Sealed with Zn


Perforated with Zn

13-Aug 1.6 10.5 5.2

17-Aug 6.3 0.4 6.3 5.8

24-Aug 7.8 0.5 6.3 5.8

31-Aug 4.2 0.5 6.3 4.2

7-Sep 4.7 1.8 5.2 4.2

14-Sep 2.6 2.1 10.5 3.7

21-Sep 2.8 2.4 20.9 3.9

28-Sep 2.6 2.1 36.6 3.1

52

Appendix E

SEM-EDX Reports

A cut piece of coupon from the bottom coupon set of each pipe was analyzed.

53

A sample of corrosion products scraped from the pipe splash zone of each pipe were

analyzed.

54

Middle coupon of sealed freely corroding pipe

Element Wt% At%

CK 06.03 15.61

OK 20.78 40.41

MgK 01.88 02.41

SK 03.09 03.00

ClK 00.59 00.52

CaK 01.79 01.39

FeK 65.84 36.67

55

Middle coupon of sealed cathodically protected pipe

Element Wt% At%

CK 14.04 34.61

OK 15.63 28.93

MgK 01.10 01.34

AlK 00.57 00.63

SK 02.99 02.76

ClK 05.23 04.37

ZnK 60.43 27.37

56

Middle coupon of perforated freely corroding pipe

Element Wt% At%

CK 07.75 18.72

OK 24.74 44.84

MgK 00.87 01.03

SiK 00.74 00.77

SK 00.51 00.46

CaK 01.04 00.75

FeK 64.35 33.42

57

Middle coupon of perforated cathodically protected pipe

Element Wt% At% CK 25.55 43.14

OK 27.10 34.35

NaK 02.31 02.03

MgK 00.65 00.54

AlK 00.31 00.23

SiK 00.74 00.53

PK 00.68 00.45

SK 00.64 00.41

ClK 00.42 00.24

CaK 20.79 10.52

FeK 20.81 07.56

58

Bottom coupon of sealed freely corroding pipe

Element Wt% At%

CK 21.19 35.47

OK 33.40 41.97

MgK 05.01 04.15

SiK 00.28 00.20

SK 00.56 00.35

CaK 25.61 12.85

FeK 13.95 05.02

59

Bottom coupon of sealed cathodically protected pipe

Element Wt% At%

CK 08.39 20.26

OK 16.92 30.69

MgK 13.05 15.57

SiK 00.66 00.68

ClK 13.65 11.18

PdL 00.53 00.14

CaK 01.83 01.32

FeK 02.53 01.32

ZnK 42.44 18.84

60

Bottom coupon of the perforated freely corroding pipe

Element Wt% At%

CK 18.06 39.22

OK 14.77 24.09

NaK 00.33 00.37

MgK 01.52 01.64

SiK 00.49 00.45

SK 06.14 05.00

CaK 09.92 06.46

FeK 48.77 22.78

61

Bottom coupon of perforated cathodically protected pipe

Element Wt% At%

CK 11.48 25.19

OK 20.39 33.58

NaK 00.65 00.75

MgK 07.86 08.52

SK 04.34 03.57

ClK 01.64 01.22

CaK 10.00 06.58

FeK 43.64 20.59

62

Splash zone of sealed freely corroding pipe

Splash zone of sealed cathodically protected pipe

Element Wt% At%

CK 04.86 15.40

OK 11.00 26.19

NaK 00.75 01.25

SK 00.36 00.43

ClK 00.36 00.39

FeK 82.66 56.35

Matrix Correction ZAF

63

Splash zone of sealed cathodically protected pipe

Element Wt% At%

CK 22.59 46.45

OK 16.06 24.79

NaK 01.84 01.98

AlK 00.36 00.33

SiK 00.44 00.38

ClK 00.45 00.31

FeK 58.25 25.76

64

Splash zone of perforated freely corroding pipe

Splash zone of perforated freely corroding pipe

Element Wt% At%

CK 07.86 16.12

OK 30.94 47.61

NaK 02.72 02.91

MgK 00.67 00.68

SK 12.57 09.65

ClK 02.94 02.04

CaK 13.52 08.31

FeK 28.78 12.68

65

Splash zone of perforated cathodically protected pipe

Splash zone of perforated cathodically protected pipe

Element Wt% At%

CK 07.66 18.05

OK 23.10 40.85

MgK 02.43 02.83

AlK 00.54 00.56

SiK 02.08 02.10

SK 00.44 00.39

ClK 01.61 01.29

CaK 12.24 08.64

FeK 49.89 25.28

The Corrosion and Biofouling Characteristics of Sealed vs ...

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