Corrosion sodium sulphate

Nickel-iron alloys are more resistant than iron to attack by solutions of various salts. In alternate immersion tests in 5% sodium chloride solution Fink and De Croly determined values of 2-8, 0-25 and 0-5 g m d for alloys containing 37, 80 and 100% nickel compared with 46 g m d for iron. Corrosion rates of about 0.4 g m d are reported by Hatfield for Fe-30Ni alloy exposed to solutions containing respectively 5 Vo magnesium sulphate, 10 Vo magnesium chloride and 10% sodium sulphate the same alloy corroded at a rate of about 1.2 g m d in 5% ammonium chloride. 

Sulphur attack on nickel-chromium alloys and nickel-chromium-iron alloys can arise from contamination by deposits resulting from the combustion of solid fuels, notably high-sulphur coals and peat. This type of corrosion, which has been observed on components of aircraft, marine and industrial gas turbines and air heaters, has been associated with the presence of metal-sulphate and particularly sodium sulphate arising directly from the fuel or perhaps by reaction between sodium chloride from the environment with sulphur in the fuel. Since such fuels are burned with an excess of air, corrosion occurs under conditions that are nominally oxidising although the deposits themselves may produce locally reducing conditions. 

Oxidation tests on Nimonic 90A, in which sodium chloride was introduced into the atmosphere, showed that this constituent produces a significant deterioration in the protective nature of the normally adherent film. Although under certain service conditions the presence of sodium chloride is likely, this is not always so, and thus the general applicability of the results of laboratory tests in sodium sulphate and mixtures involving sodium chloride may be questioned. Test procedures for hot-salt corrosion have been reviewed by Saunders and Nicholls who concluded that burner rig testing is the most appropriate procedure provided contaminant flux rates similar to those found in an operating turbine are used in the rig. 

While the few examples quoted provide some general guidance as to the behaviour of nickel-rich materials in contact with molten metals and salts, it cannot be over-emphasised that such behaviour can be very considerably modified by the presence of very small amounts of contaminants in the liquid media (see Sections 2.9 and 2.10). The effect of very small contents of sodium chloride on the corrosion of nickel-base alloys by sodium sulphate has been referred to previously and other reported examples involving trace amounts, particularly of gaseous impurities, underline the need for great care in interpretation of experimental results. 

Zinc in contact with wood Zinc is not generally affected by contact with seasoned wood, but oak and, more particularly, western red cedar can prove corrosive, and waters from these timbers should not drain onto zinc surfaces. Exudations from knots in unseasoned soft woods can also affect zinc while the timber is drying out. Care should be exercised when using zinc or galvanised steel in contact with preservative or fire-retardant-treated timber. Solvent-based preservatives are normally not corrosive to zinc but water-based preservatives, such as salt formulated copper-chrome-arsenic (CCA), can accelerate the rate of corrosion of zinc under moist conditions. Such preservatives are formulated from copper sulphate and sodium dichromate and when the copper chromium and arsenic are absorbed into the timber sodium sulphate remains free and under moist conditions provides an electrolyte for corrosion of the zinc. Flame retardants are frequently based on halogens which are hygroscopic and can be aggressive to zinc (see also Section 18.10). 

For use in high resistivity soils, the most common mixture is 75% gypsum, 20% bentonite and 5% sodium sulphate. This has a resistivity of approximately 50 ohm cm when saturated with moisture. It is important to realise that carbonaceous backfills are relevant to impressed current anode systems and must not be used with sacrificial anodes. A carbonaceous backfill is an electronic conductor and noble to both sacrificial anodes and steel. A galvanic cell would therefore be created causing enhanced dissolution of the anode, and eventually corrosion of the structure. 

Packaging materials Materials to be used in contact with metals should be as free as possible from corrosive salts or acid. BS 1133, Section 7 1967 gives limits for non-corrosive papers as follows chloride, 0-05% (as sodium chloride) sulphate, 0-25% (as sodium sulphate) and pH of water extract 5 -5-8 0. Where there is doubt, contact corrosion tests may be necessary in conditions simulating those in the package. 

An important industrial interest is in the corrosion of metals and ceramics by molten sodium sulphate/vanadate solutions. This is because turbines, which are usually nickel-based alloys, operating in a marine atmosphere, containing... 

The first two components are the active surfactants, whereas the other components are added for a variety of reasons. The polyphosphate chelate Ca ions which are present (with Mg ions also) in so-called hard waters and prevents them from coagulating the anionic surfactants. Zeolite powders are often used to replace phosphate because of their nutrient properties in river systems. Sodium silicate is added as a corrosion inhibitor for washing machines and also increases the pH. The pH is maintained at about 10 by the sodium carbonate. At lower pH values the acid form of the surfactants are produced and in most cases these are either insoluble or much less soluble than the sodium salt. Sodium sulphate is added to prevent caking and ensures free-flowing powder. The cellulose acts as a protective hydrophilic sheath around dispersed dirt particles and prevents re-deposition on the fabric. Foam stabilizers (non-ionic surfactants) are sometimes added to give a... 

Partial blocking effect was first identified for pure iron in contact with aerated sulphuric acid medium [55]. Corrosion of carbon steel in sodium chloride media clearly showed the porous layer effect (see Section 5.2) [74]. The same effect was found for zinc corrosion in sodium sulphate [75] and the properties of the layer which was demonstrated to be formed of an oxide/hydroxide mixture were further used for building a general kinetic model of anodic dissolution [76], usable for measurement of the corrosion rate from impedance data. 

It is the sodium-vanadate-sulphur system that is often responsible for the corrosion in oil fired equipment. Niles and Sanders [1962] suggested that reactions between sodium sulphate and vanadium pentoxide are important. The three principal reactions are ... 

Aqueous salt solutions are corrosive to stainless steel, especially at low pH. When aqueous acetic acid solutions are used as eluent, attention must therefore be paid to corrosion of the HPLC system. However, some salts, such as sodium sulphate, sodium nitrate, sodium acetate and sodium perchlorate, cause little corrosion of stainless steel even at low pH. Accordingly, these salts are a good choice for use together with acetic acid. Salts containing... 

Deslouis, C. Duprat, M. Tulet-Tournillon, C. (1984) The Cathodic Mass Transport Process During Zinc Corrosion in Neutral Aerated Sodium Sulphate Solutions. /. Electroanal Chem., Vol. 181, pp. 119-136, ISSN 0022-0728... 

Baril G and Pebere N (2001), The corrosion of pure magnesium in aerated and dearated sodium sulphate solutions . Corrosion Science, 43, 471 484. 

Chen J, Wang J, Han E, Dong J and Ke W (2007), AC impedance spectroscopy study of the corrosion behaviour of an AZ91 magnesium alloy in O.IM sodium sulphate solution , Electrochimica Acta, 52, 3299-3309. 

Rosalbino F, Angelini E., Maccio D., Saccone A., Delflno S. (2007), Influence of rare earths addition on the corrosion behaviour of Zn-5%A1 (Galfan) alloy in neutral aerated sodium sulphate solution , E/ectrachira. Acta, 52, 7107-7114. 

The addition of sodium sulphate to an ammonium perchlorate solution leads to pitting corrosion that is so severe that the use of aluminium cannot be envisaged. This has been shown with solutions containing 200 g l sodium sulphate and 300 g l ammonium perchlorate. 

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