Inhibitors pitting corrosion

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

Microbiocides may be toxic to humans therefore, care must be taken when used. When selecting the microbiocide, the field engineer can obtain pertinent information on chemicals from the service company providing the chemicals. The microbiocide selected must be compatible with the system in which it is being used. Some chemicals such as quaternary amines have dual functions one as microbiocides and the other as film-forming corrosion inhibitors. Insufficient concentrations of this type of chemical may not be enough to coat the whole surface of metal and can cause pitting corrosion. The selection must also depend on chemicals that can produce the desired control in minimum time limits and... 

Kato, M., Inoue, T., Goto, K., Ito, G. and Shimizu, Y., Effect of Dissolved Oxidising Agents and Inhibitors on Pitting Corrosion of Aluminium in Water , Aluminium, 49, 289 (1973) Richardson, J. A. and Godwin, A. W., Localised Corrosion of Stainless Steel During Food Processing , Br. Corros. J., 8, 259 (1973)... 

Hatch, G. B., Maximum Self-generated Anodic Current Density as an Inhibitor Pitting Index , III. State Water Surv., Circ. No. 91, 24 (1966) C.A., 66, 8l8l4f Herbsleb, G., Pitting Corrosion on Metals with Elearon-conductive Passive Layers , tVerksl. Korros., 17, 649 (1966) C.A., 66, 5337m ... 

Aluminium pipes Aluminium might become an important material for carrying water if its liability to pitting corrosion could be overcome. Very soft waters are difficult to accommodate when normal pipe materials are used, and it is for these that aluminium offers most promise ". The possibility of using it for domestic water pipes, however, appears at present to depend upon finding a cheap and effective inhibitor that could be added to the water, or upon the use of internally clad tube, e.g. Al-1 25 Mn alloy clad with a more anodic alloy, such as Al-lZn. Such pipes are at present mainly used for irrigation purposes. ... 

Molybdate also functions as an effective inhibitor by slowing down pitting corrosion through a mechanism of adsorption onto the pit wall. 

Electrochemical Testing. Potentlodynamlc polarization measurements provided a sensitive means of evaluating the inhibitors with respect to environmental (Cl ) corrosion protection. The results obtained from anodlcally polarizing polished 7075-T6 A1 samples are presented in Fig. 9. For the control electrolyte (O.IN Na2S0, 0.002N KCl, no inhibitor), pitting was observed almost immediately on the surface, and the aluminum showed no evidence of passivation. The addition of NTMP to the solution did not appear to protect the metal... 

Sodium nitrite is incorporated into formulations for both open and closed cooling systems and acts as an anodic inhibitor. It is a good passivator but requires a relatively high dose rate to ensure that all anodic areas within a system are protected from the risk of pitting corrosion. The dose rate has to be increased when high chlorides or sulfates are present. 

Basic premise No acid required. Organic inhibitor base, but with low Zn, Mo, P04, or inorganic mix, which adds capability for tracing, and also aids low-end, pitting corrosion control ... 

It should be mentioned that passive layers are not protective in all environments. In the presence of so-called aggressive anions, passive layers may break down locally, which leads to the formation of corrosion pits. They grow with a high local dissolution current density into the metal substrate with a serious damage of the metal within very short time. In this sense halides and some pseudo halides like SCN are effective. Chloride is of particular interest due to its presence in many environments. Pitting corrosion starts usually above a critical potential, the so-called pitting potential /i]>j. In the presence of inhibitors an upper limit, the inhibition potential Ej is observed for some metals. Both critical potentials define the potential range in which passivity may break down due to localized corrosion as indicated in Fig. 1. 

Anodic inhibitors reduce corrosion by polarizing the anodes rapidly. If insufficient anodic inhibitors are used, severe localized pitting will occur in the unprotected areas. Examples of anodic inhibitors are phosphates, silicates, nitrates, and chromates. ... 

E7.9. Pitting inhibitor effectiveness depends strongly on type and anion concentration. The thermodynamic (critical) activity, ai, required to inhibit pitting corrosion for a given activity of the aggressive anion is described by [13] ... 

Inorganic inhibitors such as chromate, nitrite, nitrates, and arsenate are very efficient for ferrous alloys. They prevent stainless steel pitting corrosion by forming a monolayer or multilayer passivating films. For chromate, the film is formed by iron oxidation (anodic... 

The hydroxide has the highest anion effectiveness to stop pitting corrosion Estimated inhibitor anion effectiveness is Perchlorate < Sulphate [Pg.718]

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