Susceptibility to 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. 

Dezincification Some brass alloys are susceptible to pitting corrosion or loss of zinc from the metal matrix. This type of corrosion usually occurs when metal is in contact with high percentages of oxygen and carbon dioxide. 

Pitting corrosion (Table 4.8) involves pit initiation (breakdown of passive film) followed by pit growth. The chloride ion induces pitting corrosion. Type 304 steel undergoes pitting more readily than Type 316 steel. The molybdenum in 316 steel is responsible for its reduced susceptibility to pitting corrosion. Type 316L steels contains... 

In Figure 20.83, determination of susceptibility to pitting corrosion was shown. The best-known example of an international environmental protection agreement that results, as a side effect, in a rearrangement of production processes and a withdrawal of products, is the Montreal agreement for the protection of the ozone layer. The prohibition of the production and use of specific chlorofluo-rocarbons that, aside from their action on the stratospheric ozone layer, are also water endangering has required rearrangements of production (Umweltbundesamt 1989). 

FIGURE 20.83 Determination of susceptibility to pitting corrosion. Schematic potentiodynamic current density versus potential curves. 

Sulphates (SO. The total sulphate ion content of the water should be maintained at less than 1 ppm for optimum corrosion protection. However, for unlined pools where water quality is difficult to control, sulphates at or below 10 ppm should give satisfactory protection. An increase in sulphate concentration results in a decrease in thickness of the protective oxide film, with a corresponding increase in susceptibility to pitting corrosion. 

Alloys of the 2XXX and 7XXX with tempers that are susceptible to intergranular corrosion, exfoliation, or stress corrosion cracking should not be used in seacoast atmospheres. Thin products should be afforded protection against perforation even if they are only susceptible to pitting corrosion. For alloys whose tempers are resistant to exfoliation and stress corrosion cracking the need for protection is dependent on two factors ... 

Pitting usually starts at material inhomogeneity sites, e.g. sulphide inclusions in the surface. Not only the sulphur content of the steel plays a role here, but also the number and distribution of sulphides [19]. Since the sulphur contents of steels have dropped constantly in ne v steel production processes, modem steels are less susceptible to pitting corrosion than comparable qualities from earlier production. 

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. 

Parts made of the steel SAE 304 (X5CrNil8-10, DIN-Mat. No. 1.4301) were damaged by crevice corrosion after less than 6 months, whereas bolts made of the same material used as fasteners for unalloyed steel or aluminium were not damaged [114]. In condenser pipes cooled with seawater, the steels of the SAE 304 series failed after brief exposure due to massive pitting and crevice corrosion. [148]. The material also reacts to addition of chlorine to prevent fouling with an increased susceptibility to pitting corrosion [149]. 

Attention In the sensitised state, these steels have an increased susceptibility to pitting corrosion and a strongly reduced resistance to intergranular stress corrosion cracking. 

Materials SCC in the liquid phase fractured samples, tested samples, test duration Corrosion rate (max.) mm/a (mpyj Susceptibility to pitting corrosion ... 

It has been reported that alloy 400 is susceptible to pitting corrosion in sea water, especially under quiescent conditions (Bo-gar and Peterson, 1985 Ali and Ambrose, 1991). Occasionally the corrosion pits stopped deepening and started growing laterally. 

Zirconium is susceptible to pitting corrosion in the presence of halide ions, especially chlorides. In pure hydrochloric acid, zirconium does not undergo pitting corrosion but if the electrolyte also contains ferric or cupric ions, the corrosion potential increases and pitting corrosion occurs. It has, for example, been shown that the presence of 500 ppm Fe -" in 20% HCl at 100 °C induces pitting corrosion in zirconium (Yau and Maguire, 1990). 

The comparison of the dissolution potentials of aluminium alloys may reach absurdity, for example, leading to a preference for alloys of the 2000 series, which have a dissolution potential far less negative, about — 650 mV, over those of the 5000 series, which have a more electronegative potential, on the order of - 800 mV (Table B.1.3). And yet the latter show excellent corrosion resistance, while alloys of the 2000 series are highly susceptible to pitting corrosion in natural environments. 

DeKiibes aprocedureto determine the repassivation potential of aluminum alloy 3003- HI4 as a measure of relative susceptibility to pitting corrosion by conducting agalvanostatic polarization See also ASTM G100 summarized later in this table. 

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