Precipitation localized corrosion

Pure tin is completely resistant to distilled water, hot or cold. Local corrosion occurs in salt solutions which do not form insoluble compounds with stannous ions (e.g. chloride, bromide, sulphate, nitrate) but is unlikely in solutions giving stable precipitates (e.g. borate, mono-hydrogen phosphate, bicarbonate, iodide) . In all solutions, oxide film growth occurs and the potential of the metal rises. Any local dissolution may not begin for several days but, once it has begun, it will continue, its presence being manifested... 

Localized corrosion of passivating metals initiates at local heterogeneities, such as inclusions and second-phase precipitates as well as grain boundaries, dislocations, flaws, or sites of mechanical damage. In the case of stainless steel surfaces, pit initiation occurs at sites of MnS inclusions. Exclusion of inclusions and precipitates, nonequilibrium... 

In the presence of chloride ions, a local breakdown of rust layers makes an anode channel for localized corrosion of underlying steel, and the chloride ions tend to accumulate in the channel as the anodic metal dissolution progresses. Assuming the ferrous chloride concentration at 1 mol dm 3 in the anode channel, we obtain the proton level at pH 4.75, where no ferrous hydroxide precipitation is expected to occur because of its solubility greater than 1 mol dm 3. The hydrated ferrous chloride produced by corrosion is then oxidized by air-oxygen in the anode channel ... 

It should be observed that, because of the current flowing from the anodic area towards the cathodic areas, theoretically some Fe ions migrate away from the corroding site and thus they do not precipitate locally as expansive oxides. This could mean that corrosion products due to macroceUs may have less expansive effect than corrosion products due to microceUs. 

The local current density within corrosion pits may be extremely large. If the precipitation of corrosion products does not occur, the metal dissolution is controlled by charge transfer and ohmic effects, and hence the corrosion process is potential dependent. This situation requires a sufficiently acidic solution to avoid the precipitation of insoluble oxides or a still not saturated or supersaturated pit electrolyte with no formation of a salt layer. Pitting at potentials close to the critical value Ep occurs usually with moderately small local current densities / c,p that, however, may increase with potential to extremely... 

Mesa attacks. They are a form of sweet corrosion. Mesa attacks involve the local removal of the protective iron carbonate film formed during sweet corrosion. This is achieved first by localized corrosion beneath the iron carbonate film and subsequent removal of the film by flow forces. The risk of mesa attacks is reduced considerably at temperatures higher than 100°C, as the rate of precipitation of iron carbonate is very high at such temperatures, leading to improved iron carbonate film protection [8]. 

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