Anodic inhibitors chromate concentration

Chromates and molybdates are effective inhibitors for zinc, and they are the basis of many conversion coatings. Since, however, chromates are anodic inhibitors, their concentration should be maintained above 0.5% to avoid the risk of pitting, which arises when inhibitors of the reaction at the anode are present in insufficient quantity. Chromates may have to be phased out in the coming decade, however, to comply with more stringent laws relating to pollution and toxic substances. 

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. 

Examples of the latter are chromates, which are reduced to Cr(III) hydroxide or oxyhydroxide on the metal surface, or polyphosphates, in which decomposition and subsequent precipitation of Ca phosphate has been suggested [8]. The precipitation reactions will depend on the local solution composition (pH, metal ion concentration) in the near-surface region of the corroding metal, which may pronouncedly deviate from that in the bulk. For instance, the production of OH in the cathodic partial reaction will raise the surface pH and thus promote the precipitation of compounds, such as Zn hydroxides, even in noticeable acidic solution. In a similar way, the pore-plugging ability of anodic inhibitors may be enhanced by reactions with local metal ion accumulations in the vicinity of active pores in a passive film. 

Chromates are anodic inhibitors. They are used associated with phosphonates at a concentration of a few milligrams per liter. TThey can reduce the corrosion rate of steel considerably in systems that operate under controlled scale-formation conditions. 

The inhibition of metal corrosion in industrial water systems was first achieved by the use of inorganic salts or their blends, including chromates (Evans, 1936 Mayne and Pryor, 1949), nitrites (Hatch, 1952), phosphates (Patterson and Jones, 1952), borates (Mercer, 1990), silicates (Lehrman and Shuldenen, 1952), zinc salts (Hatch, 1965 a) and other cations (Hinton, 1989). Additionally, chromates and nitrites were mainly applied, and from the end of the 1950s the use of polyphosphates increased (May et al., 1981 Hwa, 1971). Treatments with anodic inhibitors such as nitrites or chromates require a high initial dose and a relatively high continuous dose in order to achieve an effective passive layer on the metal surface. The concentration of chromate and nitrite can be decreased in the presence of polyphosphates and zinc ions. 

In modern practice, inhibitors are rarely used in the form of single compounds — particularly in near-neutral solutions. It is much more usual for formulations made up from two, three or more inhibitors to be employed. Three factors are responsible for this approach. Firstly, because individual inhibitors are effective with only a limited number of metals the protection of multi-metal systems requires the presence of more than one inhibitor. (Toxicity and pollution considerations frequently prevent the use of chromates as universal inhibitors.) Secondly, because of the separate advantages possessed by inhibitors of the anodic and cathodic types it is sometimes of benefit to use a formulation composed of examples from each type. This procedure often results in improved protection above that given by either type alone and makes it possible to use lower inhibitor concentrations. The third factor relates to the use of halide ions to improve the action of organic inhibitors in acid solutions. The halides are not, strictly speaking, acting as inhibitors in this sense, and their function is to assist in the adsorption of the inhibitor on to the metal surface. The second and third of these methods are often referred to as synergised treatments. 

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. 

It is well known that quite low concentrations of certain oxygen-containing anions, such as chromate, are effective inhibitors of aqueous corrosion of a number of metals. These results have been ascribed to specific adsorption of the inhibitor at anodic sites of the metal surface, or alternatively to continuous repair of the protective film. 

Experimental work on Al/MnOj primary or dry batteries was concentrated on the D-size cylindrical battery using a construction similar to the one used for the Mg/Mn02 battery (Fig. 9.3). The most successful anodes were made of a duplex metal sheet consisting of two different aluminum alloys. The inner, thicker layer was more electrochemically active, leaving the outer layer intact in the event of pitting of the inner layer. The cathode bobbin consisted of manganese dioxide and acetylene black, wetted with the electrolyte. Aqueous solutions of aluminum or chromium chloride, containing a chromate inhibitor, were the most satisfactory electrolytes. 

The relationship between additive concentration and corrosion rate raises the question of just what can be accomplished in reducing corrosion. Should complete stifling of corrosion be the goal If some small amount of corrosion is acceptable, is this then in the form of increased pitting, compared to the untreated system, thus making the situation worse than without the inhibitor This consideration is particularly important when working with anodic-type inhibitors such as chromate. 

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