Pitting seawater

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Nonuniform corrosion or pitting corrosion frequently occurs on steel structures in seawater and in soil. Nonuniform and pitting corrosion easily lead to damage in tanks, pipelines, water heaters, ships, buoys and pontoons, because these structures lose their functional efficiency when their walls are perforated (see Chapter 4). 

Pitting is a form of localized corrosion in which part of a metal surface (perhaps 1 per cent of the exposed area) is attacked. Rates of pitting penetration can be very high type 316 stainless steel in warm seawater can suffer pit penetration rates of 10 mm per year. This is a natural... 

Nickel is usually alloyed with elements including copper, chromium, molybdenum and then for strengthening and to improve corrosion resistance for specific applications. Nickel-copper alloys (and copper-nickel alloys see Section 53.5.4) are widely used for handling water. Pumps and valve bodies for fresh water, seawater and mildly acidic alkaline conditions are made from cast Ni-30% Cu type alloys. The wrought material is used for shafts and stems. In seawater contaminated with sulfide, these alloys are subject to pitting and corrosion fatigue. Ammonia contamination creates corrosion problems as for commercially pure nickel. 

Table 3.23 Effect of seawater velocity on pitting corrosion of 316S3I. Tests for 3 /z years...

Pitting corrosion always remains a worthy subject for study, particularly with reference to mechanism, and the problem conveniently divides into aspects of initiation and growth. For 6061 alloy in synthetic seawater, given sufficient time, pit initiation and growth will occur at potentials at or slightly above the repassivition potential . In an electrochemical study, it was found that chloride ions attack the passive layer as a chemical reaction partner so that the initiation process becomes one of cooperative chemical and electrochemical effects . 

Use of material with good pitting corrosion resistance is desirable in seawater and oilfleld brine (formation water) media. 

HSI anodes are subject to severe pitting by halide ions and this precludes their use in seawater or other environments in which these ions may be present in quantity. They are ideal for fresh-water applications (below 2(X)p.p.m. Cl"), although not for temperatures above 38°C. The addition of Mo or Cr to the alloy can improve performance under these conditions, with an upper limit of temperature of which may be affected by the... 

Comparative tests between HSI and HSCI in seawater at 93° C and 10-8Am showed consumption rates of 8-4kg A y and 0-43 kg A y , respectively . These figures show that the consumption rate of HSI when used in seawater without the addition of chromium may approach that of steel, but because of the very deep pitting and its fragility, it is in most cases inferior to steel. However, in fresh waters HSI has a far lower corrosion rate than steel. The consumption rate of HSCI freely suspended in seawater in the current density range 10-8 to 53-8 Am increases from 0-33 kg A y at 10-8Am to 0-48 kg A" y at 53-8Am Direct burial in seawater silt or mud will also increase the consumption rate, with values of 0-7kg A y at 8-5 Am increasing to 0-94 kg A " y at 23-4 Am . 

Figure 21 Pit penetration as a function of time for pure copper exposed to natural seawater. The pit penetration rate can be estimated from the slope of the line. While the initial pit penetration rate is rapid, it decreases substantially after the first year. (Data from C. R. Southwell, J. D. Bultman, A. L. Alexander, Materials Performance, 15 (1976).)...

Seawater. The recommended alloys for exposure to unpolluted seawater are 5XXX wrought alloys and 356.0 and 514.0 cast alloys27. It is likely that some pitting corrosion may occur and rates of the order of 3-6 pm/yr during the first year and 0.8-1.5 pm/yr averaged over a 10-yr period have been observed. The depth of seawater exposure of samples appears to be of no relevance. 

Nickel is resistant to chloride-induced SCC, but subject to caustic cracking in aerated solutions under high stress. Nickel is highly resistant to corrosion in natural fresh water and flowing seawater. Pitting occurs under stagnant or crevice conditions. 

Some typical applications of the alloys are in propeller shafts, propellers, pump impeller blades, casings, condenser tubes and heat exchanger tubes. The corrosion rate in flowing seawater is <0.025 mm/yr, but can pit under stagnant water. Alloy 400 is immune to chloride SCC. 

Acid solutions, excluding hydrochloric e.g., phosphoric, sulhiric, most conditions, many organics Neutral solutions, e.g., many non oxidizing salt solutions, chlorides, sulfates Caustic and mild alkalies, excluding ammonium hydroxide Neutral or alkaline solutions, e.g, persulfates, peroxides, chromates Pitting media, acid ferric chloride solutions Freshwater supplies Seawater Steam Furnace gases with incidental sulfur content ... 

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