Chromium precipitation/dissolution reactions

Species may differ by oxidation state for example, manganese(II) and (IV) iron(II) and (III) and chromium(III) and (VI). Oxidation state is influenced by the redox potential. Mobility is affected because oxidation state influences precipitation-dissolution reactions and also toxicity in the case of heavy metals. 

The toxicological impact of chromium is dependent upon its oxidation state (chromium (III) or chromium (VI)) which, in turn, is controlled by the oxidation-reduction potential and pH. Within this framework, the hydrolytic speciation of chromium(III) is important since contamination of water by chromium is widespread (Ball and Nordstrom, 1998). Solution conditions, including ionic strength, affect the speciation of chromium and its adsorp-tion/desorption and precipitation/dissolution reactions (Ball and Nordstrom, 1998). 

For metals such as chromium and alloys such as stainless steel, the plot of potential versus corrosion rate above the range is shown in Figure 20.67. Figure 20.68 shows a sudden sharp drop in corrosion above some critical potential. Despite a high level of anode polarization above V, the corrosion rate drops precipitously due to the formation of a thin, protective oxide film as a barrier to the anodic dissolution reaction. Resistance to corrosion above is termed passivity. The drop in corrosion rate above can be as much as 10 to 10 times below the maximum rate in the active state. With increasing corrosion potential, the low corrosion rate remains constant until at a relatively high potential the passive film break down, and the normal increase in corrosion rate resumes in a transpassive region. 

Chrome baths always contain a source of hexavalent chromium ion (e.g., chromate, dichromate, or chromic acid) and an acid to produce a low pH which usually is in the range of 0-3. A source of fluoride ions is also usually present. These fluoride ions will attack the original (natural) aluminum oxide film, exposing the base metal substrate to the bath solution. Fluoride also prevents the aluminum ions (which are released by the dissolution of the oxide layer) from precipitating by forming complex ions. The fluoride concenfration is critical. If the concentration is too low, a conversion layer will not form because of the failure of the fluoride to attack the natural oxide layer, while too high a concentfa-tion results in poor adherence of the coating due to reaction of the fluoride with the aluminum metal substrate. 

Oxide formation leads to a decrease in the overall oxidation rate, according to Eq. (3). The value of in this equation [which is the same as in the crack propagation rate Eq. (5)] varies with the alloy chemistry [e.g., chromium content for a denuded grain boundary of type 304 stainless steel, or sulfur content for low-alloy steels (Fig. 11)], electrode potential, and anionic activity, and this can be related to e.g., solid-state oxide growth, dissolution-precipitation reactions, and oxide breakdown [1,11]. Thus, all of the parameters in Eq. (5), apart fi om can be quantified for the crack tip system, which can, in turn, be related to de able or measurable bulk system conditions. 

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