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Scale Formation and Corrosion in Industrial Boiler

Scale Formation

The scale is formed mainly by calcium (and magnesium) salts present in water. Sodium salts are highly soluble and do not form scales.

Calcium sulfate and calcium carbonate are the principal scale formers. Calcium sulfate is the most dangerous as it forms very hard and dense scales and are difficult to remove. The scales formed by calcium carbonate are relatively softer. Magnesium sulfate and magnesium chloride are highly soluble and hence do not form scales. However magnesium hydroxide forms a sludge which is highly undesirable.

Silica

The other important scale former is silica. While silica is not a troublesome constituent in low pressure boilers, it is the most unwanted substance in water used for high pressure boilers. It forms hard, glassy scales in boilers (and on turbine blades in power generating equipment). The silica scale formation is attributed to the mechanical carry over as well as its volatility (or solubility) in high pressure steam. Silica alone or in combination with aluminium, iron, calcium, magnesium or sodium forms scales, and these scales are collectively termed as silica scales. Suspended matter, turbidity and sediment in boiler water are highly detrimental as they form both scale and sludge. Therefore the turbid waters should be clarified, (filtered if necessary) and then passed on to boilers.

Effects of scale formation

The scale is mainly formed in the feed water piping and boiler tubes. Since scale is deposited inside the piping system, the dia of the pipe is reduced which results in the reduction of flow of water. Due to scale formation, the transfer of heat from the hot gases to water is also reduced. On the contrary, the metal is overheated and may even result in rupture of tubes. The effects of scale become more severe as the boiler pressure increases.

Corrosion

Corrosion is a phenomenon of eating away of boiler metal. Corrosion is the major reason for the deterioration and failure of the equipments leading to expensive shutdowns and / or replacements.

The corrosiveness of water is due to its low pH, dissolved gases and some of the salts present in it. Needless to say that low pH induces corrosion. (However, under boiler conditions, concentrated alkalis contributing to high pH are also reported to attack steel).

Dissolved oxygen and carbon-dioxide are the principal gases present in water inducing and accelerating corrosion.

  • A solution of oxygen in water is very corrosive to metals such as iron, steel, galvanized iron and brass.
  • Low pH values accelerate the rate of this dissolved oxygen corrosion. (High pH values tend to retard it and that is why the water is made sufficiently alkaline for use in boilers).
  • Carbon dioxide by itself is corrosive in nature and is also an accelerating factor in dissolved oxygen corrosion.
  • Condensate waters are more corrosive due to the presence of oxygen and carbon dioxide.
  • Corrosiveness of water is also attributed to the presence of other gaseous impurities such as ammonia, hydrogen sulfide and sulfur dioxide.
  • Among the salts present in water, magnesium chloride and sulfate are noted for their corrosiveness. Magnesium chloride especially is corrosive and the action seems to be rather cyclic in nature. The following reactions are reported to take place inside the boiler. Magnesium chloride decomposes into magnesium hydroxide and hydrochloric acid. The latter attacks the boiler metal forming ferrous chloride. This ferrous chloride subsequently reacts with the magnesium hydroxide to form ferrous hydroxide and magnesium chloride.

Corrosion inside a boiler leads to the pitting or perforation of the metal and loss of mechanical strength due to thinning of metal and / or development of cracks. 

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