Chromate

Chromate is probably the single most effective corrosion inhibitor available. In use for many years, chromate is an anodic inhibitor, which forms a highly passive film of ferric and chromic oxides on metal surfaces.

Specifically, in cooling systems operated in the pH range of 6.5-8.0, the reaction of chromate with ferrous ion is sometimes formulated by combining two half-reactions as follows:

OH- + Fe(OH)₂——————> Fe(OH)₃ + e-3e- + CrO4^-2 + 4H₂O ——————> Cr(OH)₃ + 5OH-CrO₄ ^2- + 3Fe(OH)₂ + 4H₂O ————> Cr(OH)₃ + 3Fe(OH)₃ + 20H-

The two hydrous oxides formed subsequently dehydrate to a mixture of Cr₂03 and Fe203, which is said to constitute a protective film of oxides over anodic sites on the surface of the metal. To a lesser extent, chromates can also prevent cathodic depolarization by adsorption of the chromate on the cathodic surface, thereby preventing the diffusion of dissolved oxygen.

Historically, chromate was initially used in cooling systems at a system pH of 6.0-7.0. At these control parameters, the potential for calcium carbonate precipitation is minimized and corrosion protection afforded by chromate maximized. The primary problem encountered with this treatment approach centers around chromate's toxicity and its environmental impact when discharged into nature's waters. However, initial attempts to simply lower the chromate concentration below 200 ppm resulted in a corrosive pitting attack on the metal surface. It was found that if chromate were combined with other inhibitors, particularly cathodic types (e.g. zinc and phosphate), the chromate level could be reduced to 20-30 ppm Cr04 with better results than obtained at 200-300 ppm Cr04 used alone. An additional advantage of these synergized chromate treatments is the margin of safety provided against pitting attack should the chromate be momentarily underfed. Chromate/zinc treatments will be discussed further in this section.

Even though chromate treatment programs are used less frequently today, there are still cooling systems where their use is applicable and auxiliary chromate destruct equipment can be used, if necessary, to ensure compliance with discharge regulations.

As was stated, the anodic passivation that chromate provides is excellent and is the standard to which all other anodic inhibitors are compared. However, chloride ions, and to a lesser extent sulfate ions, are capable of penetrating the passive film set up by chromate to form active anodic sites. For this reason, it is necessary to increase chromate concentrations with increasing concentrations of these aggressive ions. In such chromate deficient areas as under deposits or in crevices, there is a propensity for accelerated attack.

In general, the hexavalent form of chromate used for corrosion control in industrial cooling water systems does not in itself pose a potential for deposit formation. However, in the presence of oils, greases, H₂S and other reducing agents, chromates can be reduced to the trivalent form, a state that is not only ineffective for corrosion inhibition but also forms deposits in the system.