Corrosion and safety
Definition:
- 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.