Corrosion control chromates

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). 

Strontium Chromate. Strontium chromate [7789-06-2] SrCrO, is made by precipitation of a water-soluble chromate solution using a strontium salt or of chromic acid using a strontium hydroxide solution. It has a specific gravity of 3.84 and is used as alow toxicity, yellow pigment and as an anticorrosive primer for zinc, magnesium, alurninum, and alloys used in aircraft manufacture (8) (see Corrosion and corrosion control). 

Hexavalent chromate [Cr(VI)] is still used within the industry to meet critical high corrosion control and other metal surface finishing requirements. Cr(VI) is toxic and its control generates a hazardous, costly waste. 

Oxygen, carbon dioxide and various chemicals used to reduce scaling can cause corrosion. Corrosion control is provided largely by the use of inhibitors such as chromates, polyphosphates, silicates and alkalies. 

The first methods of cooling tower corrosion control involved adding several hundred parts per million of sodium chromate, as chromate is capable of excellent anodic corrosion control at these dosages. However, these early programs were both inefficient and expensive. The advent of synergized zinc chromate-polyphosphate treatments not only made corrosion control more... 

Polyphosphates are also used in cooling systems to attain sufficient corrosion control. Cooling towers are operated in a pH range of 6.0 to 7.5 to provide optimum stability for the polyphosphate. The feasibility of cooling tower operation at higher pH levels, in which the potential for corrosion is decreased, has increased the popularity of low-chromate programs. 

The toxicity of chromate treatment has led to the development of alternative anodic inhibitors. Orthophosphate forms an iron phosphate film that protects the surface fi om corrosion attack, but the layer is less adherent and therefore is not so long lasting, as chromate derived protection. At the same time orthophosphate corrosion control can be made effective provided the system is properly managed. Because of the biological nutrient properties of orthophosphates it is likely that their use will additionally involve the application of a biocide. 

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. 

Naturally occurring oxide films on most metals do not usually provide optimum corrosion protection, and this may be modified or replaced to provide a further means of corrosion control. Common examples are the anodizing of aluminium alloys or the chromating of aluminium, zinc, cadmium or magnesium. With anodizing, the natural oxide film on the aluminium is thickened electrolytieally by up to 5 p.m. Chromating, described in detail later in the chapter, replaces the existing metal oxide film with a mixed chromium/metal oxide film of better corrosion resistance. 

SECM has also been recently used to investigate the growth of conducting polymers deposited onto metal surfaces both chemically [73] and electrochemically [10] from solution. Our own interest in conducting polymers stems from their potential as alternatives to chromate conversion coatings for corrosion control of active metals and metal alloys. Specifically, we are investigating the use of hydroxybenzene sulfonates as electrochemical mediators for the electrodeposition of polypyrrole... 

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. 

The use of soluble inhibitors as a means of controlling bimetallic corrosion presents many technical problems. Apart from the fact that this method is limited in applicability to recirculating systems, efficient anodic inhibitors, such as chromates, are frequently quite specific in their action and so certain bimetallic couples, such as the Al-Cu couple in chloride solutions, are... 

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