Corrosion inhibitors inorganic

Although much weaker than the parent compound isocyanic acid (pK = 3.7), CA is sufftciendy acidic to form salts. Many inorganic and organic salts of CA have been reported (11). Lead and 2iuc cyanurates are usehil as corrosion inhibitors (20). The 1 1 adduct, melamine cyanurate [37640-57-6] has achieved commercial importance. 

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. 

The primary types of corrosion inhibitor treatments employed are generally based on inorganic chemicals such as sodium nitrite (together with combinations of borate, silicate, molybdate, and phosphate) and the addition of even 2 to 3 pints (0.95-1.4 liters) to a boiler can immediately raise the TDS in the BW to a level at which priming can occur. Secondary problems include an associated rise in the level of BW suspended solids and sludge. 

Where water softening is provided and there is no reduction in system water TDS, treatments are primarily based on inorganic corrosion inhibitor blends (nitrite, molybdate, etc.). Under these circumstances, there is no benefit in using an expensive organic oxygen scavenger to keep the TDS level low, and a common chemical such as catalyzed sodium sulfite may be used. 

The downside of the inorganic oxidizing cleaning method is that the oxidizers are very aggressive and heavy pitting may occur unless the corrosion inhibitor is particularly effective and is present at the right concentration. 

The inorganic nitrite used as a corrosion inhibitor in aqueous alkylene glycol or polyoxyalkylene glycol solutions can be replaced with polyoxyalkylene amines [1263,1264]. Such polyoxyalkylene amines impart corrosion inhibition to the liquid in contact with the metal and the metal in contact with the vapors of the aqueous composition. Aqueous compositions containing the glycol and the polyoxyalkylene amine also exhibit a low foaming tendency. 

Anodically colored electrochromic inorganic films, 6 579-580 Anodic cleaning, 9 783, 785 Anodic (passivating) corrosion inhibitors, 26 144... 

P. Boden and D. Kingerley, Inorganic chemicals as metallic corrosion inhibitors. In Insights into Speciality Inorganic Chemicals (D. Thompson, ed.). Royal Society of Chemistry, Cambridge, 1995. 

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. 

Organic primers formulated with corrosion inhibitors are typically applied to pretreated metal surfaces to protect the surfaces prior to adhesive bonding and during environmental exposure. Pike [7-11] found that inorganic primers, such as sec-butyl aluminum alkoxide, improved the durability of aluminum-epoxy bonds when applied to both porous and nonporous aluminum oxide surfaces. It was shown that the effective thickness of the inorganic primer was directly related to the degree of oxide porosity and the depth of the porous oxide layer resulting from the normally used pretreatments for aluminum [10,11]. 

Where recirculating cooling water LSI =< 1.5, the level of actives typically required in the water is 8 to 10 ppm (assuming some inorganic corrosion inhibitors are also present). 

Current treatment solutions containing corrosive inorganic salts usually also contain corrosion inhibitors such as sodium dichromate or ammonium thiocyanate or are formulated to a more neutral pH (60). However, soluble-salt-treated wood in contact with metals should not be exposed to high relative humidities for prolonged periods. The treatment chemicals can attack and deteriorate metal fasteners. The corrosion products in turn deteriorate the wood. For example, under humid conditions, ammonium sulfate will attack the zinc and iron of galvanized punched-steel nail plates used in trusses (58). Alkaline and acidic areas are developed in the wood next to the attacked metal fastener, and cause degradation of the wood (58,63). 

Fig. 12.49. Action of corrosion inhibitors, (a) Anodic inhibitors. Examples Chromate, nitrite, molybdate, tungstate, orthophosphate, silicate, benzoate, (b) Cathodic inhibitors. Examples Ca(HC03)2, ZnS04, Cr2(S04)3, NiS04 phosphate, aminoethylene phosphate (AMP), Ag3+, Sb3+ (on iron) Hg (on zinc), (c) Mixed inhibitors. Examples organic inhibitors containing nitrogen and/or sulfur (e.g., amines, triazoles, thiazoles, alkylthioureas) inorganic inhibitors (e.g., arsenite, arsenate, selenate). (Reprinted from G. Ranglen, Corrosion of Metals, p. 165,1985 with permission from Chapman Hall.)...

Corrosion inhibitors are organic or inorganic species added to the solution in low concentration and that reduce the rate of corrosion. Inhibition can function in three different ways ... 

Add efficient inhibitors for certain systems since it is believed to assist the formation of a stronger, more stable or more readily repaired passive film. (Miller)24 Phosphates and other inorganic or organic corrosion inhibitors, used in fairly corrosive surroundings, decrease the effects of the SCC. A minimal critical concentration of some oxidizing inhibitors such as nitrites is absolutely necessary to avoid pitting. 

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