Caustic corrosion or corrosion resulting from the presence of sodium hydroxide can occur in boilers. It was previously stated that maintaining elevated hydroxyl ion concentrations is desirable for both steel and copper surfaces. This is true, but there is a range of conditions (pH values) above which the presence of free hydroxyl ions can be damaging to these materials.
Sodium hydroxide (NaOH) is one of the major chemical additives used in the treatment of boiler water. The purpose of sodium hydroxide is to maintain the hydroxyl ion concentration in the optimum range for the formation of good protective magnetite on steel surfaces. Its other role is to help in the formation of non-adherent sludge instead of scale when hardness enters the boiler water.
Excessive amounts of sodium hydroxide can, however, lead to corrosion. This is particularly true of ultra-high pressure boilers. If there is too much sodium hydroxide present at a steel surface, this chemical can react with the steel to form a soluble material which can then precipitate as a loose, porous magnetite deposit. The following reactions illustrate the manner in which it can occur:
Fe+2NaOH — Na2FeO2+H2
3Na2FeO2+4H2O — 6NaOH+Fe3O4
The normal concentrations of sodium hydroxide maintained in boilers are not harmful. However, it is possible for sodium hydroxide to concentrate in localized areas of boilers, thereby leading to localized corrosion. This occurs when heavy layers of deposits form on the boiler tubing, causing sodium hydroxide to be concentrated under the deposits at the metal surface.
Caustic corrosion can also occur when boiler tube surfaces become steam blanketed because of either excessive boiling or separation of steam and water in horizontal or inclined tubes.
Excessive boiling can result from very high heat transfer rates which can occur when burners are misaligned in a furnace and the flames impinge on the boiler tubing. In this case, boiler water containing sodium hydroxide can splash onto the steam blanketed surface and as the water boils off the sodium hydroxide concentrations can become excessive.
A similar effect sometimes occurs in the horizontal or inclined tubing if there is insufficient mass velocity in the tubing to keep the steam and water well mixed. If the water separates and flows along the bottom of the tube and the steam along the top of the tube, water can splash onto the hot dry upper surface. As the liquid boils from the droplet on the hot upper surface, excessive sodium hydroxide concentrations can occur.
Because caustic corrosion is a very significant problem in ultra-high pressure boilers, the free hydroxide must be eliminated from the boiler water by using the co-ordinated phosphate method of treatment.