Ignore sources of atomic hydrogen that can promote hydrogen embrittlement at your peril!
Understanding the common sources of hydrogen can help assist in preventing the premature failure caused by the insidious effects of hydrogen embrittlement.
As detailed in last month’s technical tip, Hydrogen Embrittlement is defined as a permanent loss of ductility in a metal or alloy caused by hydrogen in combination with local tensile stress. Nascent (atomic) hydrogen diffuses through the material to high energy sites such as stress concentrating features, flaws, dislocations, and other areas of lattice distortion where it combines to form molecular hydrogen. This results in a build-up of pressure of sufficient magnitude to cause local microscopic deformation and cracking – which typically occurs on grain boundaries.
The hydrogen typically stems from two sources namely i) corrosion, and ii) manufacturing processes such as cleaning, pickling, heat treatment, carburizing, phosphating, brazing and welding, as well as forming and cutting operations (where lubricants break down). Significant risk is associated with acid pickling used to remove scale prior to plating/electro plating operations. In components with a high risk of hydrogen embrittlement (such as high strength steels and bolts) alkaline descaling or mechanical cleaning processes should be employed. Hydrogen is also evolved during the electroplating process especially when the plating solution becomes depleted.
In plated materials, the plating often traps the hydrogen against the steel and when load is applied the trapped hydrogen migrates to surface defects or areas of material discontinuity (such as carbides where it concentrates, building pressure and causing small micro cracks).
The greater the concentration (partial pressure) of hydrogen the greater the likelihood of hydrogen embrittlement. The amount of hydrogen accumulated in the manufacturing process (and installation) is cumulative and stringent control of all phases of manufacture need to be applied to prevent hydrogen embrittlement.
Once the material has been embrittled it is not reversible. Every effort should be made to control the evolution of hydrogen and/or to get rid of hydrogen that may have inadvertently been introduced, by baking at temperatures between 190 and 220 deg C as soon as possible after the hydrogen exposure!
This is the last Tech Tip for the year - we trust you have found the tips informative and they have been useful in helping improve integrity/prevent failure. Please note we will be closed from 16 December 2015 and will be back on 4 January 2016.
The Origen team wishes you and yours a stress free festive season and a crackingly good New Year!
Published in Technical Tips by Origen Engineering Solutions on 1 December 2015