Seawater pitting corrosion

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). 

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. 

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. 

Consider a pit formed in a piece of aluminum that is in contact with seawater. As we shall show, the pH of the solution inside a pit can become quite low, leading to an increased rate of corrosion, which further lowers the pH, and so on. Thus, pitting corrosion can be considered to be an autocatalytic process, with its rate increasing with time. 

In a case of pitting corrosion on an aluminium sheet in seawater, the largest pit depth is 200 pm after 2 months. What will be the maximum depth expected after 1 year After 10 years ... 

Corrosion pitting in seawater is observed largely above 40% Ni because pit growth is favored by passive-active cells (see Section 6.5), and such cells can operate only when the alloy is passive—that is, in the range of high nickel compositions. Practically, this distinction is observed in the specification of materials for seawater condenser tubes in which pitting attack must be rigorously avoided. The cupro nickel alloys are used (10-30% Ni), but not Monel (70% Ni-Cu). 

Critical pitting potentials of 0.38 V (S.H.E.) in IV NaCl and 0.45 V in 0.1 V NaCl [6] indicate that the metal is vulnerable to pitting in seawater. It undergoes intergranular S.C.C. in anhydrous methyl or ethyl alcohol containing HCl, but not when a small amount of water is added [7]. This behavior, similar to that of commercial titanium, suggests that stress may not be necessary and that the failure is perhaps better described as intergranular corrosion. 

The pitting potential of a passive alloy in aerated seawater at pH 8 and at 40 °C is equal to 0.80 V relative to the saturated calomel electrode. Is there a risk of pitting corrosion if this alloy is exposed to aerated seawater at this temperature ... 

Anderson, D. B., Statistical Aspects of Crevice Corrosion in Seawater, Galvanic and Pitting Corrosion—Field and Laboratory Studies, ASTM STP 576, ASTM International, West Conshohocken, PA, 1976, pp. 231-242. 

CMoride ions in acpieous solutions represent a specific corrosive agent for pitting corrosion on passive materials. In seawater as well, the chloride ions are responsible for pitting corrosion. The starting point for pitting corrosion is a locally increased adsorption of chloride ions to damaged or weak points in the passivation layer. 

Figure 9 shows resistance to pitting corrosion of the high-alloyed steels and nickel alloys most frequently used in seawater [23]. The pitting resistance equivalent is listed for comparison [5]. 

Figure 9 Resistance to pitting corrosion of high-alloyed steels and nickel alloys in seawater based on the pitting resistance equivalent (PRE) [5, 23]...

Numerous tests - including natural seawater exposure tests - have demonstrated that a small chromium addition reduces the corrosion rates considerably without rendering steels more susceptible to pitting corrosion. In the upper part of Figure 15, the influence of chromium on seawater corrosion of a structural steel is presented [47]. Accordingly, only 0.5% and 1% Cr have a significant effect and reduce mass losses by 35%/65% compared to chromium-free steel. Improvements from higher chromium contents above this level are then relatively small. 

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