Effect of Seawater Level on Corrosion Behavior of Different

EFFECT OF SEAWATER LEVEL ON CORROSION BEHAVIOR OF DIFFERENT ALLOYS1

Saleh A. Al-Fozan and Anees U. Malik

Saline Water Desalination Research Institute Saline Water Conversion Corporation (SWCC) P.O.Box 8328, Al-Jubail 31951, Saudi Arabia.

E-mail: [email protected]

ABSTRACT

Seawater is one of the most corroded and most abundant naturally occurring

electrolyte, covering about 70% of the earth's surface. The corrosivity of the seawater

is reflected by the fact that most of the common structural metals and alloys are

attacked by this liquid or its surrounding environments.

The laboratory immersion test technique has been used to evaluate the effect of

seawater level on corrosion behavior of different alloys. In three sets of experiments,

carbon steels (G1010 and 1020), 304 SS, 316L SS, 90/10 Cu/Ni, 70/30 Cu/Ni, Incoloy

825 and Inconel 625 were utilized.

The specimens were fixed at three locations namely: above seawater surface, semi

submerged in seawater and fully submerged in seawater. The experiments have been

carried out at room temperature with very slow seawater movement (12 lit/hrs). The

exposure time was varied between one to two years.

In this study the corrosion behavior of different alloys has been evaluated based on

exposure location. Beside corrosion rate calculations, the localized attack has also

been evaluated.

1. INTRODUCTION Seawater is one of the most corroded and most abundant naturally occurring electrolyte.

The corrosivity of the seawater is reflected by the fact that most of the common

structural metals and alloys are attacked by this liquid or its surrounding environments.

1 This paper has been presented at International Desalination Association (IDA) World Congress Conference held at Singapore in 2005.

The seawater environments can be divided into five zones namely: subsoil,

continuously submerged, tidal, splash zone above high tidal and atmospheric zone [1].

The corrosion behavior of metals and alloys differ from one zone to another. In splash

zone the stainless steels have usually satisfactory performance while, the carbon and

low alloy steels do not. Anderson and Ross had found that the austenitic grades

performed much better than martensitic and ferritic grades [2]. The Ni, Cu and P

alloyed steels were found to be much more resistant than carbon steel in splash zone

[3]. Also, it was found that Mn, P and Al had measurable influence on corrosion rates

of low carbon steels under tidal exposure. After 5 years exposure test it was found that

the rate of attack in splash zone was much higher than the atmosphere and deep

submerged zones [4].

Oxygen, biological activities, pollution, temperature, salinity and velocity are the major

factors which affected the corrosion behavior of materials in the submerged zone. The

corrosion behavior of conventional stainless steels indicates that pitting and crevice

corrosion are the most usual mode of attack in this zone [5]. The results of several

studies indicate that the alloys susceptible to corrosion will perforate within first years

of exposure, whereas the resistant alloys will exhibit no pitting attack for the full

exposure time of 8 to 18 months [6]. The depth of pitting at ambient air saturated

seawater after 16 months exposure was found to be 2.4 mm on 316L SS, while after 18

months exposure to deaerated seawater at 105oC, the pit depth was only 0.12 mm [7].

The corrosion rate and pitting potential of stainless steels in seawater are the functions

of Cr and Ni content, also the presence of alloys elements such Co, Mo and N has

significant and beneficial influence on the pitting and crevice corrosion resistance of

stainless steels [8]. The decease in corrosion rate of low alloy steels over the longer

periods was more gradual than that which was observed over the first year of exposure

[9]. To prevent pitting and crevice corrosion in austenitic stainless steel, 8% of

molybdenum content is required whereas for ferritic stainless steels, the amount is

approximately 25% chromium with 3.3% molybdenum [10].

In this paper the effect of different seawater levels on the corrosion behavior of some

structural alloys has been studied. Also, the influence of exposure time on corrosion

rate is evaluated.

2. EXPERIMENTAL METHOD AND MATERIALS

Rectangular test specimens of different alloys with 50 × 20 × 2 mm in dimension were

utilized in the experimental work. Carbon steel (G1010 and 1020), Austenitic stainless

steel (AISI 304, and 316l SS), copper based alloys (90/10 Cu/Ni and 70/30 Cu/Ni) and

nickel based alloys (Incoloy 825 and Inconel 625) were used during the experiments.

The chemical compositions of the alloys are given in Table (1).

The specimens were exposed to seawater under different levels (above seawater level,

semi-submerged and immersed fully in seawater). Figure 1 shows the schematic

diagram of the experimental set-up. The seawater flow was kept very low (12/lit/hrs) at

room temperature. Weight loss coupon method technique had been used to determine

the corrosion rates. The test specimens were abraded on 400 grit SiC paper.

3. RESULT AND DISCUSSION

Tables (2) through (9) show the corrosion rates of the alloys used at different exposure

sites and under different exposure time. The results show that the carbon steel G1020

has the maximum corrosion rate in all three locations whilst the inconel alloy 625

shows the lowest corrosion rate. Under fully immersed condition, there is a decrease in

corrosion rates with increasing of exposure time for all the test alloys. The reason for

this phenomenon may differ from alloy to alloy mainly depending on chemical

composition of the exposed alloy. For carbon steel, the corrosion product will be built

up on the surface which would reduce the mass transfer of oxygen and other agents to

the metal surface resulting in the reduction of the kinetic of the cathodic reactions.

Therefore, the increase in the corrosion product thickness will decrease the corrosion

rate. The corrosion product on the copper nickel alloys surface has the ability to protect

these alloys. This corrosion film is usually quickly formed on the alloy surface but it

takes considerably long time to reach a steady state. This corrosion film is usually

enriched by iron oxide and nickel oxide and therefore, the corrosion rate of copper

based alloys may be effected by iron content in the alloy composition and/or ferrous

ions in water stream. The decease in corrosion rate of stainless steel with time could be

attributed to the thickening of passive film on the alloy surface.

The corrosion rate of alloys at the above seawater level showed two different behaviors

compared with submerged location of the same alloy. In some cases the corrosion rate

at the above water level was higher than at the submerged location of the same alloy

whereas other alloys show different behavior. For 316L SS the corrosion rate at the

above water level was higher than the submerged location. The higher corrosion rate

above seawater is due to higher O2 concentration. A higher O2 concentration favors

cathodic reaction which tends to increase the rate of pit propagation after initial pitting.

For other alloys the corrosion rate at the above water level was lower than the

submerged location of the same alloy. The increasing of pitting density and pit depth on

304 SS in submerged location compared with above water level location resulted in

increased corrosion rate of this alloy at submerged location. This phenomenon was also

observed on alloy 825 but to a lesser extent. The increase of corrosion rate of alloy 625

in submerged location compared with above water level location could be attributed to

the thickening of passive film in submerged location. The alloy 625 was found free

from the pitting in all of the two locations.

The corrosion rates of the specimens at semi-submerged location for all the tests are

higher than the other locations. The most effected area in test specimens was found at

water line zone. This attack could be due to the formation of differential aeration cell.

Due to low oxygen solubility in water the oxygen concentration will be higher above

the water surface. The pitting with different depths was found in all the alloys surface

except alloy 625. The maximum depth of attack was found to follow the sequence (in

decreasing order):

Carbon steel > 304 S> 316L SS > 90/10 Cu/Ni> 70/30 Cu/Ni> alloy 825.

Table (10) shows the pit depth corrosion results on stainless steel alloys. Figure (2)

through Figure (4) shows the relation between molybdenum content and corrosion rate

in three locations namely, above seawater level, semi-submerged and submerged,

respectively. In all the three figures the corrosion rate decreases with increasing

molybdenum content. In semi-submerged and submerged conditions, the rate of

decreasing corrosion rate of Incoloy 825 as compared with 316L SS could be attributed

to the high content of chromium in Incoloy 825.

Figure (5) through Figure (7) show the relation between molybdenum content and pit

depth for different alloys exposed at above seawater level, semi-submerged and

submerged respectively. In semi-submerged area the high level of chromium content

and presence of titanium to Incoloy 825 are responsible for decreasing of pit depth as

compare with 316L SS which has similar level of molybdenum content.

The Pitting Resistance Equivalent with nitrogen consideration (PREN) was calculated

for stainless steel and nickel based alloys following the equation:

PREN = % Cr + 3.3 × % Mo + 16 × % N

The relations between PREN and pit depth at three locations (above seawater level,

semi-submerged and fully merged) are shown in Figure (8) through Figure (10)

respectively. In submerged zone at low values of PREN, small change in PREN will

give significant change in pitting resistance at propagation stage. But at high value of

PREN, the change in pit depth is small with changing of PREN. At semi-submerged and

above seawater level locations, significant improvement in pitting resistance is found at

propagation stage with increasing PREN.

4. CONCLUSIONS

1. The corrosion rate of carbon steel (G1020) are the highest at all the three

locations, namely, above the seawater, partial submerged and fully

submerged. While the Inconel alloy 625 has the lowest corrosion rate at all

three locations.

2. The most severe corrosion for alloys used are observed at partially

submerged zone as compared to other locations.

3. The carbon steel, 304 SS and 316L SS have been markedly affected by

water line corrosion.

4. With increase of nickel content in copper base alloys the resistance to water

line corrosion increases.

5. Titanium addition to Incoloy 825 has beneficial effect at semi-submerged

location in minimizing the pitting depth.

5. REFERENCES

1. F. L. LaQue, "Marine corrosion and prevention", p. 116, John Wiley &sons, 1975.

2. D. B. Anderson and R. W. Ross Jr., "Proection of steel piling in marine splash and spray zone-Metallic sheathing concept", Proceeding 4th international congress on marine corrosion and fouling, France, pp. 461-473, 1976.

3. C. P. Larrablee, "Corrosion Resistant experimental steels for marine application", 14 (11), 501-504, 1958.

4. A. A. Humbles, "The cathodic protection of steel in seawater", Corrosion, September, 1949.

5. W. K. Boyd and F. W. Fink, "Corrosion of metals in marine environments", Metal & ceramic information center report, MCIC-78-37, March, 1978.

6. F. M. Reinhart and J. F. Jenkins, "Corrosion of materials in surface seawater after 12 and 18 months of exposure", Final report, NCEL-TN-1213 AD743872, January, 1972.

7. J. W. Oldfield and B. Todd, "Corrosion consideration in selected metals for flash chamber", Desalination, no. 32, (1-3), 365, 1979.

8. A. U. Malik, N. A. Siddiqi, S. Ahmed and I. N. Andijani, Corrosion science, 37 (10), 1521-1535, 1995.

9. C. P. larrabee, "Corrosion resistance of high strength low alloy steel as influenced by composition and environment", Corrosion, 9, 259-371, 1953.

10. H. P. Hack, "Corrosion behavior of 45 Mo-containing stainless steels in seawater", Corrosion 82, paper no. 65, March, 1982.

Composition Element Material Fe Cr Ni Mo Cu Mn C S N Si P Others

G1010 Bal 0.72 0.06 0.45 0.07 0.52 0.1 0.016 --- 0.13 0.01 (Al-0.004) G1020 Bal --- --- --- --- 0.92 0.22 0.014 --- 0.24 0.019 --- 304 SS Bal 18.28 8.13 0.17 0.19 1.48 0.047 0.01 0.076 0.49 0.019 (Co-0.14)

316L SS Bal 16.92 11.31 2.05 --- 1.04 0.021 0.004 0.048 0.46 0.028 --- 90/10 Cu/Ni

1.41 --- 9.93 --- 88.18 0.273 0.001 0.002 --- --- 0.002 (Pb-0.004) & (Zn-0.155)

70/30 Cu/Ni

0.53 --- 30 --- 68.81 0.51 0.02 --- --- 0.01 0.003 (Pb-0.01) & (Zn-0.08)

Incoloy 825

30.41 23.34 40.22 2.74 1.76 0.41 0.02 0.001 ------ 0.17 --- (Al-0.4) & (Ti-0.86)

Inconel 625

1.58 21.75 63.8 8.86 --- 0.03 0.025 0.0005 0.04 0.005 (Ti-0.19) & (Al-0.13) & (Nb-3.43) & (Co-0.01)

Table 1. Alloys chemical composition

Corrosion Rate (mpy) Duration (days)Type of exposure 247 500 750

Above seawater level 0.4287 0.4228 0.8164

Semi-submerged 2.3508 2.5154 2.8541

Fully submerged 1.5649 1.4983 1.2683

Table 2. Corrosion rate of carbon steel (G1010) in different exposure positions.



Semi-submerged 2.9278 3.1259 3.0717

Fully submerged 2.4364 2.416 2.1693

Table 3. Corrosion rate of carbon steel (G1020) in different exposure positions



Semi-submerged 0.02878 0.03145 0.0301

Fully submerged 0.01442 0.0219 0.0187

Table 4. Corrosion rate of AISI 304SS in different exposure positions



Semi-submerged 0.01409 0.02497 0.01893

Fully submerged 0.00961 0.00255 0.001963

Table 5. Corrosion rate of AISI 316L SS in different exposure positions

Corrosion Rate (mpy) Duration (days) Type of exposure 198 401

Above seawater level 0.01233 0.00548 Semi-submerged 0.11326 0.1009 Fully submerged 0.145 0.0821

Table 6. Corrosion rate of 90/10 Cu/Ni in different exposure positions



Table 7. Corrosion rate of 70/30 Cu/Ni in different exposure positions



Table 8. Corrosion rate of Incoloy (Alloy 825) in different exposure positions

Corrosion Rate (mpy) Duration (days)

Type of exposure 252 450 Above seawater level 0.00247 0.001193

Semi-submerged 0.00312 0.00301 Fully submerged 0.0027 0.00188

Table 9. Corrosion rate of Inconel (Alloy 625) in different exposure positions

304 SS 316L SS Incoloy 825 Inconel 625 Pit depth (mm)

Exposure Time (days) Type of Exposure 199 402 740 199 402 740 252 450 252 450

Above sea water level 0.31 0.47 0.93 0.18 0.36 0.65 0.05 0.27 0 0 Semi-Submerged 0.73 1.08 1.38 0.58 0.81 .15 0.17 0.36 0 0 Submerged 0.52 1.38 1.54 0.21 0.39 0.81 0.08 0.29 0 0

Table 10. Pitting depth results of stainless steels at different locations and exposure time

Seawater inlet

Seawater outlet

Specimens

Seawater Level

Specimen holder

Figure 1. Test container and specimen holder

00.0020.004

0.0060.0080.010.012

0.0140.016

0246810

Mo Content

Cor

rosi

on R

ate

(mpy

)

Figure 2. Effect of Molybdenum Content on Corrosion Rate of Stainless Steel for Specimens Located above Seawater Level

0

0.005

0.01

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0.02

0.025

0.03

0.035

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Mo ContentC

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Rat

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py)

Figure 3. Effect of Molybdenum Content on Corrosion Rate of Stainless Steel for Specimens Located in Semi-Submerged Area

0

0.005

0.01

0.015

0.02

0.025

0246810

Mo Content

Cor

rosi

on R

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(mpy

Figure 4. Effect of Molybdenum Content on Corrosion Rate of Stainless Steel for Specimens Located in Submerged Area

00.050.10.150.20.250.30.350.40.450.5

0246810

M o C ontent

Pit D

epth

(mm

)

Figure 5. Effect of Molybdenum Content on Pitting Depth of Stainless Steel for Specimens Located above Seawater Level

0

0.2

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mFigure 6. Effect of Molybdenum Content on Pitting Depth of Stainless Steel

for Specimens Located in Semi-Submerged Area.

0

0.2

0.4

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0.8

1

1.2

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Figure 7. Effect of Molybdenum Content on Pitting Depth of Stainless Steel for Specimens Located in Submerged Area

0

0.050.10.150.20.250.3

0.350.40.450.5

0102030405060

PREn

Figure 8. Effect of PREN Content on Pitting Depth of Stainless Steel for Specimens Located above Seawater Level

0

0.2

0.4

0.6

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0102030405060

PREnP

ittiin

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Figure 9. Effect of PREN Content on Pitting Depth of Stainless Steel for

Specimens Located in Semi-Submerged Area

0

0.2

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0102030405060

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Pitt

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m

Figure 10. Effect of PREN Content on Pitting Depth of Stainless Steel for Specimens Located in Submerged Area

Effect of Seawater Level on Corrosion Behavior of Different

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