Inhibition, corrosion

Inhibition is a method of corrosion protection by molecules that are adsorbed on the metal surface and reduce the rate of either metal dissolution (anodic inhibition) or the rate of the 

The adsorption of the inhibitor on the metal surface follows similar rules as were discussed for deposition additives in Chapter 9. 

The selection and use of corrosion inhibitors is an important field of applied corrosion research and details on the formulation of inhibitor systems are private. In publications only general information can be found.  

Different inhibitors are used for different metals. Besides iron and unalloyed steel, copper aluminum, and zinc have to be protected. One problem is that a system often contains metal combinations and inhibitors for two or even more metals must be combined. Most inhibitors are used in cooling systems. There are open as well as closed systems. Each system demands different measures and inhibitors. 

Some examples of typical combinations of inhibitors in closed systems are 

5-Alkoxybenzotriazoles are effective corrosion inhibitors of copper and copper alloy 90EUP397455). The anticorrosion of benzotriazole on copper has been studied by surface-enhanced Raman spectroscopy, elUpsometry, and electrochemical techniques 86MI 401-01). 

Benzotriazole has also been used as an additive in anticorrosive coatings for silver layered on plastic film 89JAP8909733). An anticorrosive, electromagnetic wave-shielding coating containing tolyltriazole has been developed for aluminum 91EUP437979). 

Theoretically, most inhibitors only selectively inhibit the corrosion of certain metals. Coolants developed for aluminium and cast iron may not always be suitable for magnesium alloys. Nevertheless, the possibility eannot be excluded that some inhibitors used in particular commercial coolants for aluminium and iron-based alloys may fortuitously also have some inhibition effect on magnesium. 

Slavcheva and Schmitt [38] evaluated the corrosion inhibition effect of two 

9 Corrosion rates of magnesium in 33 vol.% basic ethylene glycol solution with various additions of KF for 14 days [27]. 

EIS measurements further show that the addition of KF into ethylene glycol leads to a strikingly enhanced Rp but decreased Q. The reduced Q and improved Rp can be associated with the formation of a three-dimensional film on the magnesium surface. Polarization curve measurements also confirm that the phase film formed on the magnesium surface is responsible for the reduced corrosion rate of magnesium. After addition of KF, the anodic polarization current is significantly reduced to a very low value and a low current-plateau is seen where the anodic current is almost independent of the polarization potential. 

The inhibition effect of fluorides has also recently been observed on AZ91 in an ethylene glycol solution [30]. However, the effect is not as evident as on pure magnesium. This could be due to the content of alnminium in the alloy. Fluorides cannot inhibit the dissolution of aluminium. 

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Detergent Additives. Diesel engine deposits ate most troublesome in the fuel dehvery system, ie, the fuel pump and both fuel side and combustion side of the injectors. Small clearances and high pressures mean that even small amounts of deposits have the potential to cause maldistribution and poor atomization in the combustion chamber. The same types of additives used in gasoline ate used in diesel fuel. Low molecular weight amines can also provide some corrosion inhibition as well as some color stabilization. Whereas detergents have been shown to be effective in certain tests, the benefit in widespread use is not fully agreed upon (77). 

Oil field uses are primarily imidazolines for surfactant and corrosion inhibition (see Petroleum). Besides the lubrication market for metal salts, the miscellaneous market is comprised of free acids used ia concrete additives, motor oil lubricants, and asphalt-paving applications (47) (see Asphalt Lubrication AND lubricants). Naphthenic acid has also been studied ia ore flotation for recovery of rare-earth metals (48) (see Flotation Lanthanides). 

Pigment Dispersion. AMP is used widely as a pigment dispersant for water-based paints and paper coatings. In small amounts, it efficiently disperses pigments and improves pH stabiUty, viscosity, corrosion inhibition, and odor (13). When AMP is used in conjunction with other surfactants, enhanced performance is obtained with less of these ingredients in the dispersion. 

Copolymers with acrylonitrile, butadiene, isoprene, acrylates, piperjiene, styrene, and polyethylene have been studied. The high cost of sorbic acid as a monomer has prevented large-scale uses. The abiUty of sorbic acid to polymerize, particularly on metallic surfaces, has been used to explain its corrosion inhibition for steel, iron, and nickel (14). 

Sulfonic acid salts have found widespread use in the area of corrosion inhibition. Lubrizol Corporation produces a wide variety of sulfonic acids, particularly in the form of magnesium salts, for use in lubricant formulations, anticorrosion coatings, greases, and resins (146,147). Petroleum sulfonates are used in epoxy resin elastomers to improve anticorrosion properties of coatings and sealants (qv) (148,149). 

Sulfuiized and sulfurchlorinated unsaturated compounds and meicaptans are used as lubricant additives (antiwear, friction modification, load-carrying, extreme pressure and temperature, corrosion inhibition, and antioxidants), refinery catalyst regeneration compounds, steel processing (annealing) aids, and vulcanization catalysts (see Lubrication and lubricants). 

Petroleum sulfonates are widely used as solubilizers, dispersants (qv), emulsifiers, and corrosion inhibitors (see Corrosion and corrosion inhibitors). More recentiy, they have emerged as the principal surfactant associated with expanding operations in enhanced oil recovery (66). Alkaline-earth salts of petroleum sulfonates are used in large volumes as additives in lubricating fluids for sludge dispersion, detergency, corrosion inhibition, and micellar solubilization of water. The chemistry and properties of petroleum sulfonates have been described (67,68). Principal U.S. manufacturers include Exxon and Shell, which produce natural petroleum sulfonates, and Pilot, which produces synthetics. 

Corrosion Inhibition. Another important property of antifreeze solutions is the corrosion protection they provide. Most cooling systems contain varied materials of constmction including multiple metals, elastomeric materials, and rigid polymeric materials. The antifreeze chosen must contain corrosion inhibitors that are compatible with all the materials in a system. Additionally, the fluid and its corrosion inhibitor package must be suitable for the operating temperatures and conditions of the system. 

Metal- Working and Hydraulic Fluids. In the preparation of fluids for metal-working and hydrauflcs, the trend has been to replace organic-based materials with aqueous-based materials. Neodecanoic acid has found apphcation in these newer fluids as a corrosion inhibitor and a viscosity improver. For example, neodecanoic acid is used in an aqueous hydrauflc fluid concentrate for corrosion inhibition and improved antiwear properties (101), in the preparation of a thickened aqueous hydrauflc fluid to reduce viscosity loss (102), and in a water-soluble metal working oil to reduce corrosion (103). In a similar vein, neodecanoic acid has been used in antifreeze concentrates for corrosion inhibition (104). 

Table 12. Chemical Compositions and Analytical Specifications for Chromate Corrosion Inhibiting Pigments...

Zinc yellow became an important corrosion-inhibiting pigment for aircraft during World War II. However, the war production rate of 11,000 t/yr has not since been reached. Now, 2inc yellow is widely used for corrosion inhibition on auto bodies, light metals, and steel, and in combination with red lead and ferric oxide for stmctural steel painting. 

The derivatives used in corrosion inhibitor formulations for down-hole use constitute a significant industrial appHcation for polyamines. Again, mono- and bisarnidoamines, imidazolines, and polyamides made from the higher polyamines are the popular choices. The products made from DETA and fatty acids have been widely used (308). A wide variety of other polyamine-based, corrosion inhibiting derivatives have been developed, generally incorporating some form of oil-soluble or od-dispersible residue. Sulfur and its derivatives are also used in these polyamine-based corrosion inhibitors on... 

Other dimer acid markets include intermediates for nitriles, amines and diisocyanates. Dimers are also used in polyurethanes, in corrosion inhibition uses other than for downweU equipment, as a "mildness" additive for metal-working lubricants, and in fiber glass manufacture. 

Figure 5.1 Effect of oxygen concentration on corrosion of mild steel in slowly moving water containing 165 ppm CaCl2 48-hour test, 25°C. [Courtesy of H. H. Uhlig, D. N. Triadis, and M. Stern, Effect of Oxygen, Chlorides, and Calcium Ion on Corrosion Inhibition of Iron by Polyphosphates, J. Electrochem. Soc. 102, p. 60 (1955). Reprinted with permission by The Electrochemical Society, Inc. ]...

Chemical corrosion inhibition can reduce all forms of corrosion including dealloying. In particular, filmers such as tolyltriazole are effective in reducing corrosion of yellow metals. 

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