Alkaline solutions corrosion

The addition of small amounts of alloying materials greatly improves corrosion resistance to atmospheric environments but does not have much effect against liquid corrosives. The alloying elements produce a tight, dense adherent rust film, but in acid or alkaline solutions corrosion is about equivalent to that of carbon steel. However, the greater strength permits thinner walls in process equipment made from low-alloy steel. 

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

In neutral solutions, icorr is in the range of a few pA cm on Al. Stationary conditions will be reached when the oxide formation is commensurate with the corrosion, d remains constant while the metal underneath the passive film slowly dissolves [87]. This can be studied in the steady state. The ITR (15) called passive corrosion takes place as long as the solubility product is not yet reached [16]. Because of the increase of solubility in acid and alkaline solutions, corrosion is enhanced in acid and alkaline solutions. This will be explained by Eq. (60) in Sect. 3.2.4.I. 

In alkaline solutions, corrosion of steel is controlled by the rate of oxygen diffusion through the precipitated corrosion product (usually ferrous hydroxide, Fe(OH)2), so corrosion rates are low. Steel is easily passivated in alkaline solutions. Amphoteric metals such as aluminum, zinc, and lead corrode slowly at low alkali concentrations, but above pH 9.0 their rates are very high and inhibitors are required. 

Activated tertiary amines such as triethanolamine (TEA) and methyl diethanolamine (MDEA) have gained wide acceptance for CO2 removal. These materials require very low regeneration energy because of weak CO2 amine adduct formation, and do not form carbamates or other corrosive compounds (53). Hybrid CO2 removal systems, such as MDEA —sulfolane—water and DIPA—sulfolane—water, where DIPA is diisopropylamine, are aqueous alkaline solutions in a nonaqueous solvent, and are normally used in tandem with other systems for residual clean-up. Extensive data on the solubiUty of acid gases in amine solutions are available (55,56). 

Vitreous silica is susceptible to attack by alkaline solutions, especially at higher concentrations and temperatures. For 5% NaOH at 95°C, although craving may be evident, surface corrosion is only 10 p.m after 24 h (87). For 45 wt % NaOH at 200°C, dissolution proceeds at 0.54 mm /h (88). The corrosion rates in other alkaline solutions are Hsted in Table 3. Alkaline-earth ions inhibit alkaline solution attack on vitreous siUca. Their presence leads to the formation of hydrated metal siUcate films which protect the glass surface (90). 

Dichloroethylene is usually shipped ia 208-L (55 gal) and 112-L (30 gal) steel dmms. Because of the corrosive products of decomposition, inhibitors are required for storage. The stabilized grades of the isomers can be used or stored ia contact with most common constmction materials, such as steel or black iron. Contact with copper or its alloys and with hot alkaline solutions should be avoided to preclude possible formation of explosive monochloroacetylene. The isomers do have explosive limits ia air (Table 1). However, the Hquid, even hot, bums with a very cool flame which self-extiaguishes unless the temperature is well above the flash poiat. A red label is required for shipping 1,2-dichloroethylene. 

Corrosion resistance of nickel allovs is superior to that of cast irons but less than that of pure nickel. There is uttle attack from neutral or alkaline solutions. Oxidizing acids such as nitric are highly detrimental. Cold, concentrated sulfuric acid can be handled. 

Zinc is attacked at high pH. However, in weakly alkaline solutions near room temperature, corrosion is actually very slight, being less than 1 mil/y (0.0254 mm/y) at a pH of 12. The corrosion rate increases rapidly at higher pH, approaching 70 mil/y (1.8 mm/y) at a pH near 14. Just as in aluminum corrosion, protection is due primarily to a stable oxide film that forms spontaneously on exposure to water. High alkalinity dissolves the oxide film, leading to rapid attack. 

Caustic Embrittlement—a form of stress corrosion cracking that occurs in steel exposed to alkaline solutions. 

The first vessel in the blowdown system is therefore an acid-hydrocarbon separator. This drum is provided with a pump to transfer disengaged acid to the spent acid tank. Disengaged liquid hydrocarbon is preferably pumped back to the process, or to slop storage or a regular non-condensible lowdown drum. The vented vapor stream from the acid-hydrocarbon separator is bubbled through a layer of caustic soda solution in a neutralizing drum and is then routed to the flare header. To avoid corrosion in the special acid blowdown system, no releases which may contain water or alkaline solutions are routed into it. 

Atz. caustic, corrosive (Calico) discbarging, discharge, -alkab, n. caustic alkali, -alka-lildsung, /. caustic alkaline solution. 

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

The numerous metals and alloys used in practice show such a wide variation in response to various anions in acid and alkaline solutions that common features are difficult to discern and a basis for predicting corrosion behaviour is not very apparent. 

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. 

AUS102A AUS202A Type 2 Type D-2 Good corrosion resistance better than Type 1 in alkaline environments As for Type 1 but preferable for alkaline solutions used in soap and plastic industries... 

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. 

On the basis of these data, nickel is considered to be a slightly noble metal, although in practice, as will be seen below, it is considerably more corrosion resistant in both acidic and alkaline solutions than would be predicted from Fig. 4.19. 

In practice, pitting of nickel and nickel alloys may be encountered if the corrosive environment contains chloride or other aggressive ions and is more liable to develop in acidic than in neutral or alkaline solutions. In acidic solutions containing high concentrations of chloride, however, passivity is likely to break down completely and corrosion to proceed more or less uniformly over the surface. For this reason nickel and those nickel alloys which rely on passivity for their corrosion resistance are not resistant to HCl. 

Nickel alloys which rely on nobility for their corrosion resistance, viz. Ni-Cu and Ni-Mo alloys in acidic solution, do not usually pit in these circumstances. It should be noted, however, that the Ni-Cu alloy Monel 400 normally forms a protective oxide film in neutral and alkaline solutions, and this is of particular significance with regard to its corrosion resistance to... 

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