Chromate corrosion inhibitors

BETZ Laboratories of Trevase, PA has developed an inorganic phosphate substitute for chromate corrosion inhibitors called Dianodic Q, which is used in 10 to 20 ppm concentrations in cooling water. 

In summary, non-chromate corrosion inhibitors can offer similar or sometimes superior corrosion protection than systems containing chromate, without some of the attendant environmental problems of chromate systems. Before substituting a new corrosion inhibitor, however, it is important to thoroughly examine the environmental implications that its use might imply. 

Maintain chromate corrosion inhibitors in cooling water at concentrations above 2,000 mg/1 (as CrO.), and hold coolant pH within the range of 8.5-9.5. 

Chromate corrosion inhibitors such as zinc chromate in primer paints have caused allergic contact dermatitis in painters (Engel and Calnan 1963 Piper 1965). Cobalt in paint dryers is a potential sensitizer (Pirila 1947 Piper 1965 Wehle 1966). 

Cook R, Elliott J, Sapp S, (2007) Electrochemical and computational screening of nontoxic organic replacements for hexavalent chromate corrosion inhibitors , in The 2007 Tri-Service Corrosion Conference (Denver, CO, December 3-7,2007). 

Taylor, S.R. and Chambers, B.D., The discovery of non-chromate corrosion inhibitors for aerospace alloys using high-throughput screening methods. Corrosion Reviews, 2007.25(5-6) 571-590. 

A number of different rare earth-based compounds were evaluated as potential replacements for strontinm chromate corrosion inhibitors in epoxy polyamide... 

Corrosion Inhibitors. Steel-reinforcing wire and rods embedded in concrete containing quinoline or quinoline chromate are less susceptible to corrosion (72) (see Corrosion and corrosion control). Treating the surface of metals with 8-hydroxyquinoline [148-24-3] makes them resistant to tarnishing and corrosion (73). Ethylene glycol-type antifreeze may contain quinoline, 2-chloro-, 4-amino-, 8-nitro-, or 8-hydroxyquinoline to prevent corrosion (74). 

The most effective way to prevent SCC in both stainless steel and brass systems is to keep the system clean and free of deposits. An effective deposit control treatment is imperative. A good corrosion inhibitor is also beneficial. Chromate and phosphate have each been used successfully to prevent the SCC of stainless steel in chloride solutions. 

Chromates are used to inhibit metal corrosion in recirculating water systems. When methanol was extensively used as an antifree2e, chromates could be successfully used as a corrosion inhibitor for cooling systems in locomotive diesels and automobiles (185). 

Corrosion inhibitors are substances which slow down or prevent corrosion when added to an environment in which a metal usually corrodes. Corrosion inhibitors are usually added to a system in small amounts either continuously or intermittently. The effectiveness of corrosion inhibitors is partiy dependent on the metals or alloys to be protected as well as the severity of the environment. For example, the main factors which must be considered before apphcation of a corrosion inhibitor to an aqueous system are the compatibility of the inhibitor and the metal(s), the salt concentration, the pH, the dissolved oxygen concentration, and the concentration of interfering species such as chlorides or metal cations. In addition, many inhibitors, most notably chromates, are toxic and environmental regulations limit use. Attention is now being given to the development of more environmentally compatible inhibitors (37). 

Water-borne primers may or may not contain chromate-based corrosion inhibitors. The limited solubility of chromate salts in water makes them less than ideal for use in water-based primers, and much work has gone into developing alternatives [36], but the performance of recently developed water based primers using strontium chromate as the corrosion inhibitor is excellent, however, and appears equivalent to the solvent-borne analogues [37]. 

The need to identify replacements for chromate-based technologies has also resulted in the development of alternatives to resin-based corrosion-inhibiting primers. These are primarily inorganic barrier coating techniques, such as sol-gel processes, which may or may not include corrosion inhibitors. These are discussed in a later section. 

Sometimes it is possible to add corrosion inhibitors to an aqueous product that is to remain in contact with tinned steel. The normal inhibitors used for protecting steel, e.g. benzoate, nitrite, chromate, etc. are suitable, provided that they are compatible with the product and that the pH is not raised above 10. In a closed container with an air-space, such inhibitors will not protect the zone above the water-line, and possibly not the water-line zone itself, against condensate. Volatile inhibitors have been used to give protection to these areas. 

To reduce the effects of corrosion, inhibitors are added, typically sodium chromate in the salt brines and sodium phosphate in the glycols. These are alkaline salts and help to counteract the effects of oxidation, but periodic checks should be taken, and borax or similar alkali added if the pH value falls below 7.0 or 7.5 [1]. 

Corrosion inhibitors are used to reduce the corrosion of surface equipment, surface casing, and the drill string by drilling and well treatment fluids. Many different corrosion inhibitors have been used. These include amine salts such as ammonium sulfite -bisulfite blends, zinc carbonate, zinc chromate, hydrated lime, fatty amine salts of alkylphosphates, cationic polar amines, ethoxylated amines, and tertiary cyclic amines. Commercial products are usually proprietary blends of chemicals. 

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