Post on 04-Feb-2018
CONDITION ASSESSMENT
AND CORROSION CONTROL
FOR
CAN-TYPE PUMP STATIONS
Douglas Sherman, P.E., Principal Consultant
Randy Nixon, Principal Consultant
2017 WEAT Biosolids and Odor and Corrosion Control Conference
OVERVIEW
• Can-type Pump Stations – Construction
and Installation
• Corrosion Mechanisms and Causes
• Identifying and Monitoring these Problems
• Corrosion Repair & Mitigation Methods
• Summary
CONSTRUCTION AND
INSTALLATION
CONSTRUCTION AND
INSTALLATION
• Prefabricated
• Epoxy coating – internal & external
• Commonly designed with CP (mag
anodes)
• Installed quickly
CORROSION MECHANISMS
AND CAUSES
CORROSION MECHANISMS
AND CAUSES
• Internal corrosion
– O2/moisture-driven
– Controlled with coatings
• External corrosion
– At breaches in coating
– Installed CP not beneficial
CORROSION MECHANISMS
AND CAUSES
• Installed by developers who don’t
know, care
• Coatings not applied with great
quality (esp. at welds)
• Backfill disrupts coating
• Dry well not electrically isolated
• Anodes not connected
UNDERSTANDING THE CORROSION
OF UNDERGROUND, FERROUS METAL
STRUCTURES & PIPING
EXTERNAL (SOIL-SIDE)
CORROSION MECHANISMS
• Electrolytic – Oxygen-driven
• Stray Current
• Graphitic – Cast iron, ductile iron
• Galvanic – Dissimilar metals, coupled
• Microbiologically Influenced Corrosion
(MIC)
EXTERNAL (SOIL-SIDE)
CORROSION MECHANISMS
• Electrolytic – Oxygen-driven
• Stray Current
• Graphitic – Cast iron, ductile iron
• Galvanic – Dissimilar metals, coupled
• Microbiologically Influenced Corrosion
(MIC)
EXTERNAL (SOIL-SIDE)
CORROSION MECHANISMS
Electrolytic – Oxygen-driven
EXTERNAL (SOIL-SIDE)
CORROSION MECHANISMS
Electrolytic – Oxygen-driven
• Microstructural composition differences in metal
surfaces produce anodic, cathodic sites
• Can also result from differential soil conditions
• Resulting potential differences
cause corrosion current to
flow from anodes to cathodes
• Corrosion rate influenced by
moisture content, oxygen,
resistivity, chemical
contamination
EXTERNAL (SOIL-SIDE)
CORROSION MECHANISMS
Stray Current
• PS near cathodically protected gas mains or 3rd-rail
DC train systems
• Driven by stray DC current leaving the steel PS to
return to circuit
• Typically involves a few
isolated or one deep pit at
the location of stray current
departure
• May be confused with
electrolytic corrosion
EXTERNAL (SOIL-SIDE)
CORROSION MECHANISMS
Anode-to-Cathode Area Relationship:
The smaller the anodic area relative to the
cathodic area, the higher the corrosion rate.
EXTERNAL (SOIL-SIDE)
CORROSION MECHANISMS
Concentration of Corrosion
at Coating Breaks & Pinholes
Rate of penetration related to
area from which metal is taken
IDENTIFYING AND
MONITORING CORROSION
IDENTIFYING AND
MONITORING CORROSION
Visually inspect walls, floor, welds
• Corrosion
• In-leakage of ground water
• If false floor, gain access and examine floor
plates
IDENTIFYING AND
MONITORING CORROSION
Ultrasonic Thickness Testing of Shell,
Floor Plates
• Need original thicknesses
• If false floor, gain access and UT floor plates
IDENTIFYING AND
MONITORING CORROSION
Structure-to-soil Potential
Measurements
• To determine if structure is corroding (-300 to
-500 mV vs. Cu-CuSO4)
• To check original cathodic protection
IDENTIFYING AND
MONITORING CORROSION
Is Corrosion Likely in the Future?
• Obtain soil samples and test/assess for
corrosivity
Soil Description Smooth, moist, brown, dense clumps with organic matter.
Resistivity as-received (Ω-cm) 1,600
Resistivity soil/water paste (Ω-cm) 1,300
Water Content (%) 15
pH 7.0
Alkalinity (mva1/kg) 1.0
Acidity (mva1/kg) N/A
Redox (mV, platinum vs. Ag/AgCl) +190
Chloride (mg/kg) 120
Sulfate (mg/kg) 387
Sulfide (yes/trace/no) no
Particle Size Analysis
% Sand
% Clay
% Silt
45.3
30.3
24.4
Soil Classification Clay Loam
SOILS CORROSIVITY ANALYSIS
Soil Test Results
SOILS CORROSIVITY ANALYSIS
IDENTIFYING AND
MONITORING CORROSION
Is Corrosion Likely in the Future?
• Obtain soil samples and test/assess for
corrosivity
• Know ground water fluctuation range and
elevations
• Check electrical isolation from connected
piping
IDENTIFYING AND
MONITORING CORROSION
Is Corrosion Likely in the Future?
• Obtain soil samples and test/assess for
corrosivity
• Know ground water fluctuation range and
elevations
• Check electrical isolation from connected
piping
• Check/measure for stray current effects
CORROSION REPAIR &
MITIGATION METHODS
CORROSION REPAIR &
MITIGATION METHODS
Localized, Deep Corrosion
• Install lap-welded patch on
interior
• Repair coating
CORROSION REPAIR &
MITIGATION METHODS
Existing Corrosion or Potential for
Future Corrosion
• Rely on original coating and CP system (rare
for original CP system to be effective)
• If in good-to-fair condition, use monitoring
approach
CORROSION REPAIR &
MITIGATION METHODS
Existing Corrosion or Potential for
Future Corrosion
• If in poor or questionable condition, install
new CP system (determine if should be
sacrificial anode or impressed current)
• If structurally compromised, excavate and
replace or make localized weld repairs; and
install CP
CORROSION REPAIR &
MITIGATION METHODS
The Maxim of Corrosion:
Anodes corrode.
Cathodes don’t.
Make a structure sufficiently cathodic,
and corrosion will stop.
CORROSION REPAIR &
MITIGATION METHODS
Steel Structure Protection with
Coating but no CP
CORROSION REPAIR &
MITIGATION METHODS
Steel Structure Protection with
Coating and CP
Sacrificial Anode
Cathodic Protection
VS
Impressed Current
Cathodic Protection
CORROSION REPAIR &
MITIGATION METHODS
CP – Sacrificial Anode System
CORROSION REPAIR &
MITIGATION METHODS
CORROSION REPAIR &
MITIGATION METHODS
CP – Impressed Current System
SIMPLIFIED GRAPHIC:
I.C.C.P. SYSTEM for
CAN PUMP STATION
TYPICAL ICCP SYSTEM LAYOUT
SUMMARY
• Reasonable condition assessment and
repair/mitigation approach without
excavation or interruption of operation.
– $15 to $25K to repair vs. couple $100K to
replace.
• For corroding (or potentially corroding)
can-type PS, if CP is properly designed
and maintained, it will extend life of PS,
and mitigate any future corrosion.
Thank you!
Any Questions?