Anodic inhibitors nitrite ions

Anodic inhibitors such as nitrites, chromates and molybdates are strong oxidizing passivators. They strengthen the protective oxide layer over the steel which otherwise would break down in the presence of chloride ions. The mechanism involves a redox reaction in which the chloride and nitrite ions engage in competing reactions the inhibitor is reduced and steel becomes oxidized to iron oxide as follows ... 

Ferrous ion, a product of the corrosion reaction in Eq. (12.6), reacts with nitrate immediately to form a barrier oxide film through Eq. (12.13). The resulting potential would then be the potential of Fe203 in water. The anodic potential shift is due to protective film surface coverage. The observed performance improvement of chromium-containing alloys suggests that the inhibitor helps stabifize both the iron and chromium oxide layer. Nitrite ions act as anodic inhibitors by increasing barrier oxide film formation rate. 

Nitrites are environmentally fiiendly anodic inhibitors. They form a passive film with ferric oxide and inhibit the corrosion of copper, nickel, and tin alloys in alkaline environments (pH levels 9-10), but aggressive ions such as chloride and sulfate ions attack and destroy the barrier film. They reduce the rate of anodic dissolution on steel as shown in Fig. 14.10 [61]. Nitrites are used only in closed systems because they oxidize to nitrates in the presence of oxygen. They are not as efficient inhibitors as chromates. 

There are two types of passivating inhibitors oxidizing anions such as chromate, nitrite, and nitrate, which can passivate steel in the absence of oxygen, and the nonoxidizing ions such as phosphate, tungstate, and molybdate, which require the presence of oxygen to passivate steel. Examples of passivators (anodic inhibitors) include chromate, nitrite, and orthophosphate (Dihua et al. 1999). 

The inhibition of metal corrosion in industrial water systems was first achieved by the use of inorganic salts or their blends, including chromates (Evans, 1936 Mayne and Pryor, 1949), nitrites (Hatch, 1952), phosphates (Patterson and Jones, 1952), borates (Mercer, 1990), silicates (Lehrman and Shuldenen, 1952), zinc salts (Hatch, 1965 a) and other cations (Hinton, 1989). Additionally, chromates and nitrites were mainly applied, and from the end of the 1950s the use of polyphosphates increased (May et al., 1981 Hwa, 1971). Treatments with anodic inhibitors such as nitrites or chromates require a high initial dose and a relatively high continuous dose in order to achieve an effective passive layer on the metal surface. The concentration of chromate and nitrite can be decreased in the presence of polyphosphates and zinc ions. 

Local pH drop is not the single cause of crevice corrosion. For example, local depletion of passivating species may be the origin of crevice corrosion as is the case for steels in cooling water with anodic inhibitors such as nitrite or chromate ions. ... 

One of the better-known corrosion-inhibitor admixtures used in attempting to control chloride-induced rebar corrosion is calcium nitrite, Ca(N02)2. The mechanism of inhibition involves nitrite ions competing with chloride ions to react with Fe + ions produced at the anode. Essentially, the nitrite ions hmit the formation of unstable iron chloride complexes and promote the formation of stable compounds that passivate the rebar surface. The following reactions have been proposed ... 

Typical inhibitors for near-neutral solutions are the anions of weak acids, some of the most important in practice being chromate, nitrite, benzoate, silicate, phosphate, and borate. Passivating oxide films on metals offer high resistance to the diffusion of metal ions, and the anodic reaction of metal dissolution is inhibited. These inhibitive anions are often referred to as anodic inhibitors, and they are more... 

---