Corrosion in alkaline solutions

Table 5.4 Corrosion in alkaline solutions" (duration of test 6 days)...

Muller-Zulow B, Kipp S, Lacmann R and Schneeweiss M A 1994 Topological aspects of iron corrosion in alkaline solution by means of scanning force microscopy (SFM) Surf. Sc/. 311 153... 

Abdel-Gaber, A.M., Khamis, E., Abo-ElDahab, H. Adeel, S. Inhibition of aluminum corrosion in alkaline solutions using natural compound. Mater. Chem. Phys. 109 2-3 (2008), pp. 297-305. 

Brignold, G. J., Electrochemical Aspects of Stress Corrosion of Steels in Alkaline Solutions , Corrosion, 28, 307 (1972)... 

Wilde, B. E. and Teterin, G. A., Anodic Dissolution of Copper-Zinc Alloys in Alkaline Solutions , Brit. Corrosion J., 2, 125 (1967)... 

The most important outcome of this theory is that the rate of dissolution should be potentially greater as the pH increases, which is in conflict with simple concepts of corrosion kinetics. However, the theory has been proved to be applicable to many systems, and BonhoeflFer and Heusler found that iron in sulphuric acid corroded at a greater rate with increase in pH, whilst Kabanov etal. found that it corroded faster in alkaline solution than in acid solution for the same electrode potential. 

Variations in pH promoted increases in corrosion potentials from acid pH levels to neutral pH thereafter, however, corrosion potentials were lowered in alkaline solutions to more active values. Decreasing pH caused a lowering of breakdown potentials in the presence of Cl and an increase in the current densities for passivation. 

Addition of nickel improves the resistance of iron and steel to corrosion by alkaline solutions. The beneficial effect is most pronounced in hot, strong caustic solutions as illustrated by the results on nickel cast irons in Table 3.37. 

Neutral and alkaline solutions Copper-base materials are resistant to alkaline solutions " over a wide range of conditions but may be appreciably attacked by strong solutions, particularly if hot. Copper/nickel alloys usually give the best results in alkaline solutions. Copper and copper alloys should be avoided if ammonia is present, owing to the danger of both general corrosion and, if components are under stress, stress corrosion. 

Ruthenium dissolves anodically in alkaline solutions, as predicted by Pourbaix but its corrosion resistance when made anodic in acid solutions is variable. Under some conditions the volatile and toxic tetroxide is evolved. Osmium is even more reactive anodically than ruthenium. 

Some metals are amphoteric. That is, they form simple cations (in acid solutions) and soluble oxyanions (in alkaline solution) only in the mid-pH range is a protective film stable. Since cathodic protection produces alkali at the structure s surface, it is important to restrict the polarisation, and thereby the amount of hydroxyl ion produced, in these cases. Thus both lead and aluminium will suffer cathodic corrosion under cathodic protection if the potential is made excessively electro negative. 

Tin is anodic to steel in alkaline solutions, the corrosion rate for a continuous coating being similar to that of pure tin, and tinned articles that are washed in aerated alkaline detergents slowly lose their coating. 

Attack by alkali solution, hydrofluoric acid and phosphoric acid A common feature of these corrosive agents is their ability to disrupt the network. Equation 18.1 shows the nature of the attack in alkaline solution where unlimited numbers of OH ions are available. This process is not encumbered by the formation of porous layers and the amount of leached matter is linearly dependent on time. Consequently the extent of attack by strong alkali is usually far greater than either acid or water attack. 

In alkaline solutions, nickel electrodes are quite common, particularly when conducting cathodic reactions (hydrogen generation, the reduction of certain organic materials). They resist corrosion under these conditions. With certain precautions (taking care not to exceed the limits of potential), nickel electrodes in alkaline solutions are useful, too, for certain anodic reactions such as the oxidation of hydrogen and methanol. 

For the corresponding equations in alkaline solutions, see Chapter 9. The metal surface attains a mixed potential corrosion potential, such that the anodic current of the metal dissolution is exactly balanced by the cathodic current of one or more reduction reactions. The corrosion potential is given by Eq. (11.41), and the corrosion current density by Eq. (11.42). 

The pH of a metalworking fluid must be kept above neutrality in order to prevent acid corrosion of the metal In vitro, acid catalyzed nitrosation is optimized at pH 3.5 (4 0) however, it has been shown that In the presence of other catalysts, aqueous solutions of amines and nitrite leads to significant yields of nitrosamines at room temperature over the pH range of 6.4 to 11.0 (41). Furthermore, C-nitro-containing, formaldehyde-releasing biocides, such as bronopol or tris nitro, exert their potential catalytic effect in alkaline solution. It would thus be desirable to determine the optimum pH for a metalworking fluid that would lead to the lowest concentration of nitrosamine possible. 

The core of the electrolyser is the cell block, which is made up of a large number of usually bipolar cells in a modular structure.8 Typical sizes of a cell block range from 1 to 800Nm3/h. (Most electrolysers sold today to laboratories, the semiconductor industry, etc., have a capacity less than 60 Nm3/h.) The biggest capacities realised are about 150 MW. An alkali solution, usually 20% to 40% potassium hydroxide (KOH), is used as the electrolyte that flows between the electrodes. In alkaline solutions, the electrodes must be resistant to corrosion, have good electronic... 

Corrosion Behavior of Epoxy and Unsaturated Polyester Resins in Alkaline Solution... 

Hatva, T. (1989) Iron and manganese in ground-water in Finland Occurrence of glacifluvial aquifers and removal by biofiltration. Publ. Water environmental research institute. Nat. Board Waters Envir., Finland, No. 4, 99 p. Haupt, S. Strehlow, H.H. (1987) Corrosion layer formation and oxide reduction of passive iron in alkaline solution A combined electrochemical and surface analytical study. Langmuir 3 837-885... 

Zhu etal. [219] have also investigated the influence of surfactants on the electrochemical behavior of zinc electrodes in alkaline solutions. In the presence of surfactants, the dendrite growth is reduced and the zinc deposit is more uniform and compact. Therefore, perfluorosurfactants can be used for decreasing the corrosion of zinc batteries. 

Zinc and zinc-coated products corrode rapidly in moisture present in the atmosphere. The corrosion process and its mechanism were studied in different media, nitrate [283], perchlorate [259], chloride ions [284], and in simulated acid rain [285]. This process was also investigated in alkaline solutions with various iron oxides or iron hydroxides [286] and in sulfuric acid with oxygen and Fe(III) ions [287]. In the solution with benzothia-zole (BTAH) [287], the protective layer of BTAH that formed on the electrode surface inhibited the Zn corrosion. 

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