Corrosion and safety


  • It is an electrochemical reaction between a metal and its environment.
  • It results in a loss of metal or weakening of it.
  • It reaches deeply, creates maintenance problems and incurs cost of loss in lacs of rupees over the years.

Causes of Corrosion:

  • It is due to oxidation at high temperature is called scaling. e.g. steam boilers.
  • Generally it takes place due to a corrosive chemical or impurity over the exposed surface.
  • Inter-granular corrosion occurs in stainless steels heated up to 500-800 °C and then exposed to corrosive conditions.
  • Galvanic corrosion happens due to current flowing between two dissimilar metals which form a galvanic cell. It occurs’ when two such metals are joined together at a weld. A typical pair is iron and copper.
  • Corrosion pitting results from electrochemical potential set up by differences of oxygen concentration inside and outside the pit.
  • The oxygen-lean part acts as anode and the metal surface as cathode.
  • Knife-line corrosion takes place between parent and weld metals. e.g. austenitic stainless steels.
  • Crevice or contact corrosion occurs at the point of contact of a metal and nonmetallic material. e.g. threaded joints.

Erosion is a type of corrosion and is caused by flow restriction or change of direction. e.g. elbows, tees, baffles, nozzles and valves and point opposite to inlet nozzle.

  • It is increased if the flow contains solid particles or by bubbles in liquids and by two phase flow.
  • Wet steam flow, air jet flashing flow and pump cavitation can cause severe erosion.

Preventive Measures:

  • It`s Prevention is of high importance as it prevents accidents and reduces cost of corroded materials.
  • Substitution of non-corrosive or less corrosive material (e.g. SS instead of MS).
  • Two compatible metals prevent or slow down the rate of it.
  • A strategically placed gasket i.e. to provide insulating material between the two metals.
  • Cathodic protection and conversion coating.
  • Crevice corrosion can be avoided by choosing materials having corrosion resistance. Stainless steels are prone to it and not recommended for such use.
  • Use of non-metallic material like plastic.
  • Applying monomolecular film (inhibitor) of grease, paint, synthetic organic coating or a plastic sheet (liner) over the surface.
  • Use of oxygen scavengers (e.g. Sodium sulphite and hydrazine) to add into boiler water to remove oxygen.
  • Use of acid pickling as corrosion inhibitors.
  • Use of heavy oils or greases, waxes dissolved in solvents or sulphonate salts dissolved in petroleum as a barrier between the environment and the metal surface.
  • Rubber lining.