Chromate anodic inhibition

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. 

Dichromate does not function as an anodic inhibitor during crevice propagation in these experiments. Dichromate is not expected to be an inhibitor at the applied potentials used here, but estimates of the potential drop place the bottom of the crevice at a potential ranging from -0.5 to -0.8 V. In this potential range, anodic inhibition might be expected, provided the chromate chloride ratio is sufficiently large. 

Overall, these results indicate that chromates inhibit corrosion by elevating the pitting potential on aluminum with respect to the corrosion potential, which decreases the probability for the formation of stable pits. In general a chromate chloride concentration ratio in excess of 0.1 is necessary to observe significant anodic 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... 

More generally, acidic depolymerization products can be leached out of many films of paint, unless suitable pigments are incorporated. Zinc dust plus zinc oxide was the original acid scavenger. Zinc phosphate, zinc ferrite, and calcium borosilicate will each exert an anodic inhibitive function at pH values exceeding 7 while zinc chromate, zinc tetroxychromate, and strontium chromate act independently of pH value (Van Eijnsbergen, 1988). In general, the pH of aqueous paint film extracts should be between 7.5 and 8.5 and certainly not below 6 or above 10. 

Given some basic information about the corrosion inhibition mechanisms of chromates, many studies have been conducted for chromate replacements. For effective replacement of hexavalent Cr, however, an inhibitor has to inhibit the oxygen reduction reaction as well as anodic dissolution/pitting, and several studies indicate that hybrid formulations seem to be the best way to do just that. Typically, in these hybrid formulations an organic oxygen reduction reaction inhibitor is included with environmentally benign anodic inhibiting anions. 

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). 

Some salts, notably chromates, dichromates, silicates, borates and cinna-mates, have marked inhibitive power and are very effective in closed-circuit water systems. Care must be taken to ensure that a sufficient quantity of such anodic inhibitors as chromates is added, as otherwise attack, though occurring at fewer points, may be more severe at these points. Chromates and dichromates have little inhibitive power in strongly acid solutions. 

A large number of electrolytic treatments of magnesium, anodic or a.c., have been developed, in which adherent white or grey films consisting of fluoride, oxide, hydroxide, aluminate or basic carbonate are deposited from alkaline solutions containing caustic alkali, alkali carbonates, phosphates, pyrophosphates, cyanides, aluminates, oxalates, silicates, borates, etc. Some films are thin, and some are relatively thick. All are more or less absorbent and act as good bases for paint, though none contributes appreciable inhibition. All can, however, absorb chromates with consequent improvement of protective efficiency. 

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

Very interesting behavior of incorporating anions can be observed when a multicomponent electrolyte is used for oxide formation. Here, anion antagonism or synergism can be observed, depending on the types of anions used. The antagonism of hydroxyl ions and acid anions has been observed in a number of cases. Konno et a/.181 have observed, in experiments on anodic alumina deterioration and hydration, that small amounts of phosphates and chromates inhibit oxide hydration by forming monolayer or two-layer films of adsorbed anions at the oxide surface. Abd-Rabbo et al.162 have observed preferential incorporation of phosphate anions from a mixture of phosphates and chromates. 

Phosphates, molybdates, and (at high pH) silicates act as anodic inhibitors much as do alkalis, except that the iron oxides/hydroxides formed on anodic sites then contain some PO43-, M0O42-, or Si044- ( basic iron phosphates, etc.). These inhibitors require the presence of 02 to produce basic iron(III) phosphate, molybdate, or silicate films, whereas oxidizing anions such as chromates and nitrites oxidize Fe2+ (aq) rapidly to insoluble iron(III) oxides on anodic sites. Dianodic inhibitors combine complementary inhibition mechanisms for example, sodium triphosphate may be used with sodium chromate, or sodium molybdate with NaN02. 

The effect of passivating films on aluminium and magnesium has been the subject of much research. By incorporating chromate/dichromate mixtures and other substances in the electrolyte, a coherent insoluble oxide film is formed which effectively inhibits further corrosion. Sealed cells with aluminium or magnesium anodes may therefore be successfully stored for several years, even at high temperatures. However, once current has been drawn from the cell, the film is broken down and rapid attack on the metal follows due to reactions such as... 

Under given conditions, passivity is attained with increasing readiness in the order iron, cobalt, nickel iron is much more difficult to render passive than is nickel in a particular electrolyte. Metals of the iron group become passive more readily in alkaline than in acid solutions, and oxidizing agents, e.g., iodate, bromate, chlorate, chromate and nitrate, favor passivity chloride ions markedly inhibit the onset of passivity. Increase of temperature increases the c.d. required for the anode to become passive under a given set of conditions. 

Anodic passivation of steel surfaces can be efficiently achieved by metal chromates. Chromates of Intermediate solubility (e.g., zinc chromate and strontium chromate) allow a compromise between mobility in the film and leaching from the film to be achieved. Chromates inhibit corrosion in aqueous systems by formation of a passivating oxide film. The effectiveness of chromate inhibitors in aqueous systems depends on the concentration of other ionic species in solution, for example, chloride. Synthetic resin composition can also significantly influence the effectiveness of chromate pigments. The effect appears to be related to the polarity of the resin (20) chromate pigments appear to be less effective in resins of low polarity. 

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