The Hendrix Group Reporter©

March 22, 1998 (Vol. VIII No. 1)

Crevice Corrosion of Nickel-Based Alloys in Seawater

Introduction

The corrosion of metals and alloys in natural and chlorinated seawater has long presented challenges for those responsible for materials selection and has been much studied. A number of alloys have bee successfully used in seawater services. Copper-nickel alloys and the 300-series austenitic stainless steels are normally considered minimum starting points for materials selection in seawater, with super austenitics, nickel-based alloys, and titanium specified for the more severe services. Crevice corrosion is a major consideration in component design and alloy selection for seawater service and will often dictate materials selection when crevices cannot be avoided. For demanding applications, where excellent overall corrosion resistance and long term equipment reliability are needed, the nickel-based alloys have been used extensively. However, even the highly alloyed nickel-based alloys, those with high molybdenum contents, are susceptible to crevice corrosion, depending on the specific seawater environment. Materials selection to combat crevice corrosion in seawater is especially complicated due to wide variations in seawater properties from location to location, difficulty in comparing various researchers' alloy ranking results, and difficulty in extrapolating test results to an anticipated service life in seawater applications. This technical note discusses some considerations in selecting nickel-based alloys for seawater applications, including environmental, design, and alloy composition influences.

Crevice Corrosion - Mechanism

The mechanism of crevice corrosion is generally accepted at this time and can be described as a four-stage process: 1,2

(1) The crevice solution becomes deoxygenated due to initial corrosion in the crevice and the diffusion rate of oxygen into the crevice is not sufficiently rapid to replace its rate of depletion due to the local cathodic process.

(2) The cathodic process moves outside the crevice, separating the anode and the cathode. Due to current flow and mass transport phenomena, chloride ions migrate to, and build up inside the crevice, causing a reduction of pH in the crevice solution.

(3) The build up of chlorides and the depressed pH create a crevice solution that causes a breakdown of the passive film on the alloy.

(4) Once the passive film is compromised, corrosion propagation commences.

Seawater as a Corrosive Medium

Composition

The composition of seawater is substantially the same worldwide, the principal variations being the ratio of water to total salt content. Total dissolved solids (TDS) vary from as low as approximately 8000 ppm (mg/L) in the Baltic Sea to as high as 60,000 ppm in bay areas of the Arabian Gulf, due to the evaporation by tropical heat in the desert air. The "nominal" dissolved solids, upon which formulae for artificial seawater are based, is about 34,500 ppm, of which about 25,000 ppm is taken to be sodium chloride. A typical seawater composition, showing major salt constituents, is given below:

Composition of Seawater (3)

Component Concentration (mg/l) % of total salt
Chloride 18,980 55.04
Bromide 65 0.19
Sulfate 2,649 7.68
Bicarbonate 140 0.41
Fluoride 1 0.00
Boric acid 26 0.07
Magnesium 1,272 3.69
Calcium 400 1.16
Strontium 13 0.04
Potassium 380 1.10
Sodium 10,556 30.61
Total 34,482 99.99

Salinity and Chlorinity

The concept of salinity is sometimes misunderstood, commonly taken as being the corrosivity of seawater. Historically, salinity has been defined as "the total weight in grams of inorganic salts in one kilogram of seawater, when all bromides and iodides are replaced by chloride equivalents and all carbonates by oxide equivalents."However, salinity is usually determined either by conductivity measurements or from the chlorinity. Chlorinity, in turn, is "the mass in grams of silver required to precipitate the halogens in 0.3285234 kilo grams of seawater." Fortunately, it is nearly equal to the mass of chloride in the same. Chlorinity is related to salinity as follows:

S % = 1.80655 Cl% [1.0]

Carbonates and Sulfates

While deep ocean waters are usually undersaturated with respect to carbonates, surface waters are usually saturated due to wave action and exposure to carbon dioxide in the atmosphere. This saturation will affect the deposition of calcium and magnesium salts in cathodic reactions during corrosion or in the application of cathodic protection. The presence of 2000 to 3000 ppm of sulfate ions in seawater of average composition facilitates corrosion under anaerobic conditions, due to the action of sulfate reducing bacteria (SRB's).

Dissolved Oxygen

The nominal saturation for dissolved oxygen (DO) in seawater is about 6-8 ppm at 25-30 oC (77-86 oF). However, the DO may go as high as 12 ppm, depending upon wave action and seawater temperature, and supersaturation may occur due to photosynthesis by phytoplankton bloom. On the other hand, the DO may decrease to practically zero due to bacterial action or biological or chemical oxygen demand. The presence or absence of DO largely overshadows everything else. Steel corrosion, for example, increases with DO despite decreasing temperature.

pH

The pH of seawater is usually in the range from 7.7 to 8.3 in surface waters, due to the buffering effects of carbonate salts. Much more acidic conditions can be produced in deep water, with pH's of 3-4 being produced by bacterial action. Simple hydrocarbons and oxygenated organic compounds produce carbon dioxide and water upon oxidation, also lowering the pH. In the latter instance, the pH would be roughly 5-6. On the other hand, living plants consume carbon dioxide by photosynthesis, where sunlight is available, raising the pH. Decomposing nitrogenous organic material (e.g., fish) will form amines and ammonia, also tending to increase the pH. It is the net result of all these possible activities, plus the effect of such specific ions as sulfides from SRB activity, which must be considered.

Localized Variations

Besides stratification effects in deep, quiet waters, temperature decreases with depth. From a surface temperature about 13oC (55oF), the temperature may diminish to about 5oC (41oF) at 3000 feet (1000 meters) depth. The DO may simultaneously drop from 6 ppm to about 1 ppm if biological or bacterial activity occurs. Otherwise, the DO will be greater because of increased oxygen solubility at the lower temperature.

Geographical Variations

Temperature extremes may vary from 2-4 oC (36-39 oF) in the polar regions to 32oC (90oF) or more in shallow waters in the tropics. The dissolved oxygen will diminish about 50% with depth in the Gulf of Mexico at 1500 feet (500 meters), but is unchanged at the same depth in the North Sea.

Characteristics

Besides compositional variations, seawater will contain varying amounts of silt, dissolved gases other than DO, and decaying animal and vegetable matter, which contribute to fouling and corrosion. Seawater, utilized for cooling purposes in chemical or other process plants, is usually taken from relatively shallow water sources or from seawells, which rarely exceed 60 feet (20 meters) in depth. Shallow estuaries or bay water are notoriously susceptible to municipal or industrial pollution. In southern climates particularly, there may be "Red Tides" (due to dinoflagellates) which tend to diminish dissolved oxygen and increase suspended solids in the form of dead marine life.

Under stagnant conditions, as when a plant cooling system is shut down during a turnaround, the seawater may become anaerobic, due to consumption of oxygen by corrosion, the effects of BOD or COD, or bacterial action (or by a combination of such factors). The action of SRB's on sulfates will produce "sour" conditions from the production of small amounts of hydrogen.

Fouling

Strictly speaking, fouling includes four separate types of phenomena. Fouling by crystalline deposits (e.g., calcareous deposits, calcium sulfate deposits) requires temperatures greater than about 90oC (190oF) and is not usually encountered in heat exchangers at the usual seawater pH. An exception would be where alkaline streams (caustic, ammonia or amines) leaked into the cooling water, raising the pH above calcium salt saturation.

Fouling from suspended solids and biological growths are normally the major concerns. The effect of sand in abrasion or erosion corrosion is well known and its capability for setting up oxygen concentration cells adequately proved. Silt from estuary waters contain not only rock particles, usually 0.05 inches (1.25 mm) or less in diameter, but also vegetable matter and possibly metallic oxides from contamination or pollution. More adherent than sand, silt contributes to fouling, plus adding to corrosion problems, not only by concentration cell effects but also contributing decomposition products. The deposition of sand and silt within the heat exchanger tubes is primarily controlled by maintaining recommended minimum velocities, usually about 5-6 fps (1.5-1.8 mps).

Bioforms

Bioforms in seawater contribute both to fouling and corrosion. Biofouling refers specifically to fouling caused by marine plants or animals when such organisms attach themselves to materials submerged in the sea. The two basic types of organisms are the "soft" plant like shines, algae and hydroids and the "hard" (shell like) barnacles, mussels, oysters, tube worms and sea squirts. The tendency of such materials to adhere to materials submerged in seawater depends on the nature of the material itself, as do the resultant manifestations of corrosion, scaling, plugging, etc. It is well known that metals and alloys that produce toxic salts (e.g., copper, lead and zinc) resist hard fouling by barnacles and the like, although they may accumulate soft fouling materials under some conditions. Controlled chlorination further aids in control of biofouling.

Chlorination Effects

The effect of chlorination on the crevice corrosion of nickel alloys has been widely investigated. The consensus is that low levels of residual chlorine (0.2 - 3.0 mg/l) reduces the rate of corrosion compared with untreated natural seawater 2, while high residual levels (~80 mg/l) significantly increase the potential for crevice corrosion 4. Determining a maximum, safe, residual chlorine level between these two values is more problematic due to the complexity of the parameters influencing crevice corrosion and the different test procedures used by various researchers. Most researchers have attempted to maintain residual chlorine levels of approximately 1-2 mg/l in their laboratory tests. 2,4,5 Reference 7 suggests 0.3-0.5 mg/l as a minimum level.

Design Effects

It is important to design out crevices in seawater exchangers coolers, particularly at flanges and particularly with seawater on the shellside of the exchanger. Vertical tube-and-shell exchangers, with water on the shellside, present corrosion problems even with traditional, low chloride (<800 ppm) inhibited, recirculating cooling water. Seawater is much more aggressive than recirculating cooling water based on its greater salinity, biomass and silt content. Ideally, seawater should be on the tubeside; however, if not possible, horizontal orientations are preferred to vertical exchangers. The horizontal orientation should increase water circulation at the tubesheet, with the associated increased heat transfer and decreased fouling and deposition.

Crevice Geometry and Surface Finish Effects

The environmental factors that influence whether crevice corrosion initiates, and at what rate it propagates are many, and include: crevice geometry, surface roughness, metal temperature, seawater velocity, seawater composition, water hardness deposition and sediments, and chlorination.

A test parameter not reproduced consistently in crevice corrosion tests between researchers, and one that we believe to have a significant influence on variability in the reported data, is crevice geometry and surface roughness, with surface roughness influencing the crevice gap. Overall, the tighter the crevice, the greater the propensity for crevice corrosion. Differences in surface roughness between test specimens has reportedly influenced the test results of several researchers, as well as material and tightening procedures used to form the crevice. 2,6,7 Where attempts have been made to measure the crevice gap in laboratory tests, they have approximated 0.2 - 2.0 microns. 1,7

Temperature Effects

Temperature plays an important role in crevice corrosion; however, its role is more complicated than one might expect. Temperature influences the kinetics of reaction rate and mass transfer. With increasing temperature, crevice corrosion initiation tendency and propagation rates increase. As a rule, when a corrosion reaction is kinetically controlled, a 10oK rise in temperature will double the corrosion rate, while under mass transfer limiting conditions the corrosion rate will double with every 30oK rise in temperature.6 With moderate water velocities the corrosion rate should be kinetically controlled. Under no, or low flow conditions, the corrosion rate should be controlled by mass transfer. Therefore, under flowing conditions, temperature plays a significant role in corrosion by increasing mass transfer.

Velocity Effects

The role of seawater velocity in the corrosion process is complex. Higher velocities provide greater heat transfer rates, resulting in cooler tube metal temperatures, while providing a fresh source of oxygen, which acts as the primary cathodic depolarizer in the corrosion process. Conversely, low or no flow conditions reduce the availability of oxygen to the cathode, but can also result in reduced heat transfer and suspended solids deposition. Several researchers have documented a correlation between seawater velocity and increased corrosion rates, versus lower corrosion rates in low-flow tests.4,6 However, taking into account all opposing influences, we believe it preferred to maintain a minimum flow at the tubesheet surface. Within practical limits (~6.6 m/s), excessive seawater velocities should not be a concern with corrosion-erosion of nickel-based alloys.

Salt Precipitation, Deposits, Bio-fouling and Sediments Effects

Hard water salt deposition, deposits, bio-growths and sediments play similar roles in corrosion overall, as well as with crevice corrosion. Their main influence is in establishing concentration cells and permitting corrosive species to collect, hide and concentrate under deposits. Deposits of all types act to accelerate corrosion and should be avoided. Carbonate salt deposition, based on its inverse solubility, can be a problem at above-ambient seawater temperatures. Available literature suggests that hard water salts can deposit out at temperatures at low as 38oC and that deposition is significant above 60oC.3 Suspended solids and sediment deposits can be minimized by maintaining minimum seawater velocities of approximately 1.5 m/s.

Nickel Alloy Performance

The nickel-based alloy family includes nickel with no other significant alloying elements, chromium-free nickel alloys, and nickel-chromium alloys, with and without molybdenum.

The essentially 100% nickel alloy (UNS NO2200) is little used in seawater, readily losing its passivity and suffering pitting and crevice corrosion. General corrosion rates as high as 8 mpy are possible in polluted seawater.3

The chromium-free nickel alloys of interest include 67 nickel- 33 copper (UNS NO4400) and 70 nickel - 28 molybdenum (UNS NO 10001). Of the two important commercial chromium-free alloys, alloy 400 has been widely used in seawater. Its general corrosion rate in quietly moving seawater ranges from approximately 0.1- 1.0 mpy.3 Pits in alloy 400 tend to self-stifle, with pits broadening with time, as opposed to growing deeper; however, care should be exercised when specifying alloy 400 for thin-wall products such as exchanger tubes. The alloy possesses excellent resistance to high-velocity seawater, exhibiting corrosion rates of approximately 0.4 mpy at velocities as high as 41 mpy 3.

Despite its high molybdenum content, the 70 Ni-28 Mo alloy is susceptible to localized corrosion in quiet and low-velocity seawater and is more expensive than other alloys with equivalent or better localized corrosion resistance.

Of the commercial nickel alloys, the ones most considered for seawater service where there is a potential for crevice corrosion include alloy 625 (NO6625), alloy C-276 (N10276), and alloy C-22 (NO6022). It is these alloys that will be principally discussed. At a much lower alloying content of elements that contribute most to crevice corrosion resistance, i.e., chromium and molybdenum, several super austenitic stainless steel alloys possess superior crevice corrosion resistance compared with lesser alloyed nickel alloys.

The critical crevice temperature (CCT) is a common parameter for ranking alloys, based on resistance to crevice corrosion. CCT data for nickel alloys is mixed, depending on the researcher, and difficult to extrapolate to actual service conditions. However, based on our review of the literature, there appears to be a real potential for failure of alloy 625 in seawater at temperatures above approximately 35oC. One researcher tested alloy 625 in 15oC seawater at a chlorination level of 2.5 mg/l with good results,5 while other researchers have observed failure of alloy 625 in 15oC to ambient temperature (~25oC) seawater.1,2,4,5 The picture is further complicated by the different values of CCT obtained for alloy 625 in a widely used standard for conducting crevice corrosion tests, ASTM G-48, Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by the Use of Ferric Chloride Solution. CCT values from 30oC to 50oC for alloy 625 have been obtained; however, a value of 30oC appears to be more realistic. CCT tests of alloy 625 conducted in a 4% NaCl solution by different researchers have resulted in reported values from 25oC 8 to 50oC.9 Attempts to correlate ASTM G-48 CCT results with service experience suggests that an alloy with a G-48 CCT value of 35oC or higher should be resistant to crevice corrosion in natural seawater at temperatures below 25oC.10 This again, suggests that alloy 625, with a G-48 CCT value of ~30oC, may fail in ambient temperature seawater.

The above data for alloy 625 is in contrast with similar data for alloy C-22, which shows much higher values for CCT in the G-48 test (55oC) and in the 4% NaCl solution (100oC).11,12 Based on reference 10, C-22 should be resistant to crevice corrosion in 350C seawater. However, as a caveat, researchers have initiated crevice corrosion in alloy C-22 at high chlorination levels in ambient temperature seawater.4 The crevice corrosion resistance of alloy C-276 is similar to that for alloy C-22.

References

1. Shaw, B.A., Moran, P.J., Gartland, P., Crevice Corrosion of a Nickel-Based Superalloy in Natural and Chlorinated Seawater, Unknown source.

2. Oldfield, J.W., Corrosion Initiation and Propagation of Nickel-Base Alloys in Severe Seawater Applications, Corrosion 95, Paper No. 266, NACE International, Houston, TX.

3. Performance of Tubular Alloy Heat Exchangers in Seawater Service in the Chemical Process Industries, MTI Publication No. 26, Materials Technology Institute, St. Louis, MO.

4. Klein, P.A., Ferrara, R.J., Kain, R.M., Crevice Corrosion of Nickel-Chromium-Molybdenum Alloys in Natural and Chlorinated Seawater, Corrosion 89, Paper No. 112, NACE International, Houston, TX.

5. Olsen, S., Nice, P.,Maligas, M., Vicic, J., Material Selection for Wellhead Equipment Exposed to Chlorinated and Natural Seawater, Corrosion 96, Paper No. 80, NACE International, Houston, TX.

6. Ijsseling, F.P., General Guidelines for Corrosion Testing of Materials for Marine Applications, Br. Corrosion J., 1989, Vol. 24, No. 1, p. 55.

7. Hack, H.P., Crevice Corrosion Behavior of Molybdenum-Containing Stainless Steel in Seawater, Materials Performance, Vol. 22, No. 6, p. 24.

8. Manning, P.E., Sridhar, N., Asphahani, A.I. New Developmental NiCrNo Alloys, Corrosion 83, Paper No. 21, NACE International, Houston, TX.

9. Haynes International, C-22 Alloy Databook.

10. Kovach, C.W., Redmond, J.D., Correlations Between the Critical Crevice Temperature "Pre-Number", and Long-Term Crevice Corrosion Data For Stainless Steels, Corrosion 93, Paper No. 267, NACE International, Houston, TX.

11. Hibner, E.L., Evaluation of Test Procedures for Critical Crevice Temperature Determination for Nickel Alloys in a Ferric Chloride Environment, Corrosion 86, Paper No. 181, NACE International, Houston, TX.

12. Corrosion Testing of Iron- and Nickel-Based Alloys: Part I: Test Methods (Revised), MTI Publication No. 46, Materials Technology Institute, St. Louis, MO.

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