Chromate cathodic inhibition

Ramsey et al. used split cell experiments to study the effect of chromates on inhibition of the anodic and cathodic reactions using Cu, Al, and Al alloy 2024-T3 in near-neutral chloride solutions (41). Split cell experiments involving... 

Anodic or cathodic passivation (introducing chemicals which change the natural oxide to make it more protective and less active). In cathodic inhibition, inhibitors predominantly form insoluble precipitates or salts due to pH rise during corrosion processes [14]. Anodically active materials [92] promote the adsorption of oxygen at the surface (e.g. chromates, molybdate), or by forming insoluble complex salts with metal ions at the anodic defect sites (e.g. phosphate, borate), known as pore plugging [14]. 

The most favorable conditions for equation 9 are temperature from 60—75°C and pH 5.8—7.0. The optimum pH depends on temperature. This reaction is quite slow and takes place in the bulk electrolyte rather than at or near the anode surface (44—46). Usually 2—5 g/L of sodium dichromate is added to the electrolysis solution. The dichromate forms a protective Cr202 film or diaphragm on the cathode surface, creating an adverse potential gradient that prevents the reduction of OCU to CU ion (44). Dichromate also serves as a buffering agent, which tends to stabilize the pH of the solution (45,46). Chromate also suppresses corrosion of steel cathodes and inhibits O2 evolution at the anode (47—51). 

Epoxy based primer systems remain the best suited for the corrosion protection of magnesium. Cathodic epoxy electrophoretic paints , chromate inhibited epoxy-polyamide primers and high temperature stoving epoxy sealers are used to provide protection up to 180°C. For higher temperature applications up to 300°C, epoxy silicone or polyimide based systems can be used. 

The primary function of a coating is to act as a barrier which isolates the underlying metal from the environment, and in certain circumstances such as an impervious continuous vitreous enamel on steel, this could be regarded as thermodynamic control. However, whereas a thick bituminous coating will act in the same way as n vitreous enamel, paint coatings are normally permeable to oxygen and water and in the case of an inhibitive primer (red lead, zinc chromate) anodic control will be significant, whilst the converse applies to a zinc-rich primer that will provide cathodic control to the substrate. 

Zinc salts rapidly generate zinc hydroxide or salt protective films on cathodic surfaces when they are added to cooling water. They are generally used in conjunction with other corrosion inhibitors, such as organophosphates. BETZ manufacturers a combination HEDP-zinc inhibitor. They have also been used with chromate systems to inhibit the chromate concentrations required. A disadvantage of zinc is its tendency to precipitate in pH environments greater than 8.0 (Roti 1985). While the toxicity of zinc to humans is far lower than that of chromium, its toxicity to marine and aquatic life is high. 

Chromates are excellent inhibitors of oxygen reduction in near neutral and alkaline solutions. In these environments, they can stifle corrosion by suppressing this cathodic partial reaction. The inhibition mechanism appears to involve reduction of Cr(VI) to Cr(III) at a metal surface and formation of Cr(III)-0-substrate metal bonds (38). This surface complex is likely to be substitutionally inert and a good blocker of oxygen reduction sites, as suggested by the exceedingly small water exchange rate constant for the first coordination sphere of Cr3+ (39). 

The efficiency of the electrolytic oxidation is limited to some extent by the reduction of chlorate at the cathode. To check this reduction as far as possible, a little chromate or dichromate is added to the bath. This is partially reduced at the cathode to tripositive chromium, which forms a coating of a chromic chromate around the cathode and thus inhibits the cathodic reduction of chlorate or hypochlorite. 

Chemical Treatment. A wide variety of chemicals and water treatments are used for corrosion control. Corrosion inhibitors usually act by forming some type of impervious layer on the metallic surface of either the anode or cathode that impedes the reaction at the electrode and thereby slows or inhibits the corrosion reaction. For example, various alkali metal hydroxides, carbonates, silicates, borates, phosphates, chromates, and nitrites promote the formation of a stable surface oxide on metals. The presence of these chemicals in the electrolyte allows any faults in the metal surface or its oxide film to be repaired. If they are used in too small a quantity as anodic inhibitors, they may promote intense local attack because they can leave a small unprotected area on the anode where the current density will be very high. This is particularly true of chromates and polyphosphates. 

In the early 1950 s, combinations of alkali chromate (an anodic inhibitor) and polyphosphate (generally accepted as cathodic) came into prominence for cooling system corrosion inhibition. The combination of chromate with phosphates proved highly efficient in comparison with straight phosphate or straight chromate, and could be used at substantially lower concentrations. 

Passivation of iron by molybdates and tungstates, both of which inhibit in the near-neutral pH range, requires dissolved oxygen [6], contrary to the situation for chromates and nitrites. In this case, dissolved oxygen may help create just enough additional cathodic area to ensure anodic passivation of the remaining restricted anode surface at the prevailing rate of reduction of MoOi or of WOi , whereas in the absence of O2, icnticai is not achieved. 

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