- ELECTROCHEMICAL CORROSION:- WHEN IRON COMES IN CONTACT WITH WATER, HYDROGEN IONS TAKES ELECTRONS FROM METAL AND BECOMES HYDROGEN WHILE OH(-) REACT WITH METAL TO FORM HYDROXIDE OF METAL. THIS LEAD TO CORROSION OF METAL. HYDROGEN ATOM THUS FORMED MAY FORM A LAYER OVER THE METAL TO PREVENT FURTHER CORROSION. IF O2 IS PRESENT IN DISSOLVED FORM THEN H2 WILL COMBINE TO FORM H2O AND IF WATER IS ACIDIC THEN ALSO H2 CAN FORM H2 GAS AND LEAVE LEADING TO FURTHER CORROSION DUE TO BREAKING UP OF LAYER OF H2 ON METAL.
- MECHANICAL STRAINING:– THIS IS NOT A CORROSION AGENT BUT DUE TO BREAKDOWN OF THE SURFACE OF METAL, PITTING TYPE OF CORROSION COULD RESULT DUE TO DIFFERENTIAL AERATION I.E IF A PORTION OF METAL BECOMES PARTIALLY INACCESSIBLE TO O2 IT BECOMES ANODIC AND CORROSION OCCURS. MAIN CAUSES ARE:- RAISING STEAM TOO RAPIDLY FROM COLD AND FLUCTUATING FEED TEMPERATURE.
- GALVANIC ACTION
- CAUSTIC EMBRITTLEMENT:– IS CAUSED DUE TO HIGH CONCENTRATION OF CAUSTIC SODA (NAOH) AND MATERIAL UNDER STRESS. IF CONCENTRATION OF 6000 GRINS/GALLON AT 300 DEGREE CELCIUS AND IF CRACK IS PRESSED IN THE PARTS OF BOILER THEN THE NAOH GETS TRAPPED INSIDE THESE CRACKS DUE TO CAPILLARY ACTION AND SINCE WATER EVAPORATES THIS GET TRAPPED THERE. THIS NAOH REACTS WITH IRON TO FORM SODIUM FERRATE WHICH IS BRITTLE IN NATURE.