Content effect on pitting corrosion

Steigerwald, R. F., Effect of Chromium Content on Pitting Behaviour of Fe-Cr Alloys , Corrosion, 22, 107 (1966)... 

The position of the water table determines the oxygen transport and hence the corrosion rate. The moisture content of soil greater than 20% is deemed to be corrosive (general corrosion of carbon steel) and the value of less than 20% was conducive to pitting corrosion.15 This observation is thought to be related to the diffusion rate of oxygen.16 The general effect of soil resistivity on the corrosivity may be denoted as ... 

Corrosion. Fink 16) of Battelle Memorial Institute has presented the results of a literature study combined with views of experts on the corrosion of metals by sea water. The study revealed a paucity of data on corrosion at elevated temperatures. The Cl ion is the chief culprit in causing corrosion, but an important factor is dissolved oxygen and it is probable that oxygen-free sea water would have very little corrosive action, at least at ordinary temperatures. Natural sea water may have very different corrosion effects from synthetic sea water because of the organic content. Fouling of the surface by organic deposits can lead to severe pitting due to concentration-cell effects. Consequently corrosion by actual water is not readily simulated in the laboratory by synthetic sea water. 

Depending on the potential of steel and chloride content in the concrete, it is possible to define different domains where pitting corrosion can or cannot initiate and propagate, and other effects may take place. These matters are further explored in Chapter 20. 

The molybdenum (Mo) content of types 316 and 317 stainless resists the onset and development of pitting. The presence of nitrogen (N), a strong austenitic former, remarkably increases resistance to pitting and crevice corrosion. The relevant effect of Cr, Mo, and N on crevice corrosion is denoted by the widely used Pitting Resistance Equivalent Number (PREN). 

Sulfide inclusions have been found to act as initiation sites for pitting corrosion of mild steels in neutral-pH solutions [45, 46]. On the other hand, sulfur content has been found to have no significant effect on corrosion rates in adds of steels containing more than 0.01% Cu [42]. 

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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