Cooperative spreading of localized corrosion on heterogeneous materials

It is believed that critical thresholds associated with localized corrosion of metals are not only associated with rapid growth of single pits but may also be associated with rapid growth in the number of pit sites. It is hypothesized that the explosive increase in the number of pit sites, the formation of pit clusters, and spreading of pits across surfaces are caused by cooperative interactions. The goal is to determine whether changes in various physical characteristics affect the extent of cooperative interactions and the associated critical thresholds. Decreasing the lateral diffusion length of aggressive species relative to the inter-inclusion spacing was found to shift the onset of rapid increases in number of pit sites, the degree of clustering, and the conventionally measured pitting potential by mitigating cooperative interactions. It was found that rapid increases in the conventionally measured pitting potential occurred when the lateral diffusion length decreased below a characteristic inter-inclusion spacing on a series of Fe-Cr-Ni-Mo alloys with different sulfur contents to control the inclusion density. Critical threshold potentials (ECrit) were found to be unaffected above a critical threshold potential, but increased rapidly below the critical threshold potential due to the mitigation of cooperative interactions. Small amounts of Ti were added to one Fe-Ni-Cr-Mo alloys to preferentially form acid resistant TiS inclusions instead of more susceptible MnS inclusions to mitigate cooperative interactions and increase ECrit when exposed to solutions that simulated cooperative interactions. MnS bearing stainless steels were found to be damaged by the same exposures and a decrease in ECrit was observed. Cooperative interactions were further extended to intergranular corrosion (IGC) of sensitized stainless steel, which contain networks of susceptible initiation sites in the form of Cr depleted grain boundaries.; Cooperative interactions were found to initiate intergranular corrosion from pits when: (a) the material was sensitized, (b) ohmic potential drop occurred, and (c) solutions were aggressive enough to support intergranular corrosion on a series of sensitized AISI 304 and 304L stainless steels. It was found that the degree of sensitization dictates the spreading of intergranular corrosion by controlling the fraction of susceptible grain boundaries.