Stress corrosion cracking techniques

Visual identification prior to failure is difficult due to the typical tightness of stress-corrosion cracks. A low-power hand lens will greatly aid determination. Crack enhancement may be achieved through the use of dye penetrants. Severe cracking may be detectable using ultrasonic, radiographic, or acoustic emission techniques. 

Stress-corrosion cracks tend to be fine, tight, and easily overlooked. Various nondestructive techniques are available to aid in the discovery of cracks, such as dye penetrant, and ultrasonic and radiographic techniques. 

Although not commonly listed as a weld defect, high-welding stress nevertheless provides an essential ingredient to stress-corrosion cracking and other failures. It differs in an important respect from other types of weld defects in that stresses cannot be visually identified or revealed by conventional nondestructive testing techniques. 

D ye penetration inspection. This is a simple technique, requiring a minimum of operator training. In the hands of a skilled operator, it is capable of detecting fine cracks such as chloride stress corrosion cracks in austenitic stainless steels and fatigue cracks. 

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

Stress relief is of little practical value as a means of preventing stress-corrosion cracking in austenitic steels, as cracking occurs at quite low stress levels even in fully softened material and it is difflcult to ensure that stresses are reduced to a safe level in a real structure. The technique can however be useful in small items but, even in this case, phase changes which reduce stress-corrosion resistance or have other deleterious effects can occur at the stress relieving temperature. 

Ugiansky G. M. and Payer, J. H., (Eds) Stress Corrosion Cracking-The Slow Strain Rate Technique, ASTM STP 665 (1979)... 

In more recent work embrittlement in water vapour-saturated air and in various aqueous solutions has been systematically examined together with the influence of strain rate, alloy composition and loading mode, all in conjunction with various metallographic techniques. The general conclusion is that stress-corrosion crack propagation in aluminium alloys under open circuit conditions is mainly caused by hydrogen embrittlement, but that there is a component of the fracture process that is caused by dissolution. The relative importance of these two processes may well vary between alloys of different composition or even between specimens of an alloy that have been heat treated differently. 

For detecting stress-corrosion cracks and estimating their depth of penetration, the ultrasonic technique and, to a lesser extent, A -radiography, have proved successful. 

This method is the simplest of all the methods and is capable of detecting surface flaws such as corrosion, contamination, surface finish and surface discontinuities on joints.47 The discontinuities on joints such as welds, seals, solder connections and adhesive bonds can be detected. General corrosion, qualitative pitting corrosion, stress-corrosion cracking, weld-heat-affected zone attack, erosion corrosion and other type of degradation can be observed by visual examination aided by microscopes with sufficient magnification. Degradation of plastics can also be detected by visual examination. Visual examination is also used in conjunction with other techniques, such as powerful microscopes. 

WILLIAM H. SMYRL is Professor of Chemical Engineering and Materials Sciences and Associate Director of the Center for Corrosion Research at the University of Minnesota. He received his Ph.D. (chemistry) at the University of California, Berkeley, and spent 3 years at the Boeing Scientific Research Laboratories and 11 years at Sandia National Laboratories. He joined the faculty of the University of Minnesota in 1984. His research interests are modeling of corrosion processes, in situ techniques for metal-metal oxide interface studies, digital impedance for faradaic analysis, stress corrosion cracking, polymer-metal interfaces, and electrochemical processes. 

A. Kawashima, A.K. Agrawal, and R.W. Staehle, Effect of Oxyanions and Chloride Ion on the Stress Corrosion Cracking Susceptibility of Admiralty Brass in Nonammonical Aqueous Solutions, Stress Corrosion Cracking The Slow Strain-Rate Technique, STP 665, G.M. Uglansky and J.H. Payer, Ed., ASTM, 1979, p 266-278... 

Careful design, good fabrication techniques and annealing processes will do much to reduce or eliminate, the problem of stress corrosion cracking. 

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