Stress Corrosion

Stress Corrosion Notes...

Introduction

Stress Corrosion is evidenced when the metal strength loss resulting from the combined stress and corrosion is greater than the effects of stress and corrosion acting separately.   The magnitude of the combined effect is a measure of the susceptibility of the material to stress corrosion.

Stress corrosion is generally evidenced as cracks giving an appearance of brittleness in a material which is otherwise has normal ductile properties.   The cracks may follow intergranular paths which grow at relatively slow velocities.   If the load is constant during the period of crack growth the cracks will eventually reach a critical size to result in material failure.

Some combinations of environment and material known to cause stress corrosion are listed below.

Material	Environment known to cause stress corrosion
Al alloys	Moist air, Sea Water, Chloride, Bromide and iodide solutions
Cu alloys	Ammonia solutions and vapours, amines, moist SO2, solutions of acetates, citrates formates, tartrates, nitrites and sodium hydroxide.
Ni alloys	Hydroxide solutions and hydrofluoric acid vapour
Ti alloys	Solutions of chlorides, bromides, and iodides, liquid N2O4, methanolic solutions and dry salts and elevated temperatures
Low strength ferritic steels	Solutions of hydroxides, nitrates, carbonates, phosphates, molybdates, acetates, cyanides, and liquid ammonia
High Strength ferritic steels	Moist air, water, aqueous and organic solutions. Susceptibility increases with increasing stregnth of materials
Stainless Steels	Solutions of chlorides , fluorides, iodides, bromides, sulphates , Phosphates, nitrates and polythionic acids

Stress corrosion does not normally occur in conditions in which the metal suffers from serious general corrosion.    It is therefore often not noticed resulting in fracture without warning.

The best methods of eliminating or minimising this problem are listed below

1. By selecting a material that is not susceptible to the environment and by ensuring that the environment is not adversely affected by cleaning etc are not detrimental.
2. By controlling the operating stresses through design and minimising stress concentrations to keep them below the critical value.   Residual stresses can be reduced by heat treatments and careful design for manufacturing.
3. By coating the material and effectively isolating the material from the environment.
4. Minimising the operating temperature - stress corrosion is very low if the temperature is maintained below a certain specific value

Links to Stress Corrosion

1. National Physical Laboratory ... downloadable guides ..Excellent
2. Corrosionist.com Stress corrosion .....Corrosion and Corrosion protection...including useful notes and relevant links
3. Aalco - Galvanizing -Electrochemical Protection of Steel  Suppliers website including lots of useful information