• Virtually no practical and engineering material is stable. The rate of decay varies depending upon the material and its environment.• The human system cannot endure – Temp above 50 C or below 20 C – Pressures above or below our atmospheres – Presence of harmful gases such as CO, H2S, and H2.• Thus, analogous to human system, engineering materials also require protection like – coatings, inhibitors, alloy addition, design procedures, maintenance, inspection and re- furnishing.
Corrosion is the disintegration of an engineered materialinto its constituent atom due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as O2.
EFFECTS OF CORROSION• Reduces Strength• Life time is reduced• Metallic properties are lost• Wastage of metal
IMPORTANCE OF CORROSION DATA 5 mpy Good corrosion resistant material 5 to 50 mpy Low corrosion resistant material 50 mpy Unsuitable as constructional material
STRESS CORROSION BRITTLE CRACKS FORM AT THE SITES OF STRESS. FAILURE CAN BE FAST. FAILURE CAN OCCUR AT STRESS LOADS FAR BELOW YIELD STRENGTH. THREE CONDITIONS REQUIRED FOR MOST COMMON KIND ARE -----METAL UNDER DISSOLVED CHLORIDE TENSILE OXYGEN ION STRESS
ERROSION CORROSION• FLOW REMOVES PROTECTIVE LAYER.• NEW POTECTIVE LAYER FORMS USING UP METAL.
MICROBIAL CORROSIONSIMILAR TO PITTING CORROSION.BACTERIA IN WATER.
ACIVE CORROSION PROTECTION• The aim of active corrosion protection is to influence the reactions which proceed during corrosion, it being possible to control not only the package contents and the corrosive agent but also the reaction itself in such a manner that corrosion is avoided..• Examples of such an approach are the development of corrosion-resistant alloys.
PERMANENT CORROSION PROTECTION• The purpose of permanent corrosion protection methods is mainly to provide protection at the place of use. The stresses presented by climatic, biotic & chemical factors are relatively slight in this situation.• For example, in factories shed areprotected from extreme variations intemperature, which is frequently thecause of condensation.