Corrosion inhibition inorganic inhibitors

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 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. 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

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 ... 

Corrosion inhibition Is a very important but still poorly understood process. The aim is to inhibit the continuous oxidation or reduction of metals. This requires insight into the electrochemistry of the process. A host of interwoven phenomena are involved, including redox reactions on the surface, in the solution and Intervention by inorganic or organic molecules. The complexity is illustrated by the fact that there is often no simple correlation between the coverage of a certain inhibitor on the surface and the extent of Inhibition. 

Corrosion in steel systems handling wet oils can be inhibited with both organic and inorganic compounds. Effective organic compounds include various amines, lecithin, and mercaptobenzo-thiazole. The inorganic inhibitors include sodium nitrite and sodium nitrate. Chromates are not used because of their instability in the presence of organic matter. 

Corrosion inhibitors in paints protect metallic surfaces from oxidation. Coating primers are used when there is continuous exposure to corrosive elements, e.g., in marine applications. Examples are coal-tar derivatives, epoxy resins and coal-tar modified epoxies. Primers that inhibit corrosion by anodic or cathodic polarization contain inorganic metallic pigments such as chromates or leads or both. Composite pigments containing calcium oxide, zinc, silica, and oxides of phosphorus and boron can also be used (Mathias 1984). Nowadays, powder paints such as polyester and epoxy powder paints can also be used for corrosion inhibition (Rose and Vance 1997). 

Zinc ions inhibit corrosion by a cathodic polarization mechanism based on the precipitation of a zinc hydroxide film at cathodic sites on the metal surface. Zinc in combination with phosphates will lead to a protective film containing zinc phosphate. Film formation is usually rapid due to the low solubility of the zinc compounds at an alkaline pH. The low solubility of zinc in alkaline solutions requires the incorporation of dispersants. The rate of film formation with cathodic inorganic inhibitors should be carefully controlled, as dangerous fouling may occur. Protective films caused by cathodic inhibition are macroscopic and often easily visible, whereas anodic inhibitors generally from very thin, hardly detectable passive films. 

Additional corrosion inhibition can be used as a way to improve control over the corrosion processes on substrates protected with metallic coatings. It is very difficult to achieve direct addition of organic or inorganic iorric inhibitors to the metal. Therefore, an alloying approach was considered to irrtroduce a sorrrce of... 

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