Testing embrittlement

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

Louis Raymond, ed., Hjdrogen Embrittlement Prevention and Control, American Society for Testing and Materials, PhUadelphia, Pa., 1988. 

S tandard Methods of Test for Hydrogen Embrittlement of Copper, ASTM B 577, American Society for Testing and Materials, Philadelphia, Pa., 1992. Welding Braying and Soldering Vol. 6, Metals Handbook, 9th ed., ASM International, Materials Park, Ohio, 1983. 

Analysis methods for hydrogen absorbed in the deposit have been described (65), and instmments are commercially available to detect hydrogen in metals. Several working tests have been devised that put plated specimens under strain and measure the time to failure. A method for cadmium-plated work has been described (66) as has a mechanical test method for evaluating treatments on AlSl 4340 Steel (67). Additional information on testing for hydrogen embrittlement is also available (68). 

The specimen may be subjected to simple bending tests to determine whether any embrittlement has occurred. 

A boiler s water may have caustic embrittling characteristics. Only a test using a U.S. Bureau of Mines Embrittlement Detector will show whether this is the case. If the water is found to be embrittling, it is advisable to add sodium nitrate inhibitor lest a weak area of the boiler be attacked. 

A prestressed roller bearing is used to detect the presence of hydrogen sulfide, but more specifically it is used to test for hydrogen embrittlement tendency of the drilling fluid. When introduced to the environment, the bearing has sufficient residual stresses to cause failure if sufficient hydrogen sulfide concentration is present. 

Although tests on smooth specimens indicate that cathodic protection of maraging steel is possible, tests on specimens with pre-existing cracks indicate a greater sensitivity to hydrogen embrittlement during cathodic polarisation . The use of cathodic protection on actual structures must therefore be applied with caution, and the application of less negative potentials than are indicated to be feasible in smooth specimen tests is to be recommended if it is assumed that structures contain crack-like defects. 

There are several classes of test for hydrogen embrittlement, according to the application. Three general types of mechanical test can be identified, together with chemical and electrochemical tests intended to determine the hydrogen content of steels or the rate of entry of hydrogen from an environment. 

While the conventional slow strain-rate test offers many benefits, it does suffer from a tendency to overstate the susceptibility of materials to hydrogen embrittlement. Thus structural steels of modest strength will fail even under conditions giving relatively low rates of hydrogen entry. This is... 

Another modification to the slow strain-rate test involves the superimposition of a low amplitude sine wave ripple on the slow uniform extension (Fig. 8.47). In effect this produces higher strain rates (which appear to be more damaging for hydrogen embrittlement), while still giving a long test duration, with adequate time for the accumulation of hydrogen in the steeps. 

The role of the stress in embrittlement and stress-corrosion processes has been examined in some detail by employing the slow strain-rate technique . Specimens of alloy 7179-T651 tested in air or in vacuum after pre-exposure to water at 70° C or in water at various potentials at ambient temperature exhibited a reversible embrittlement in excess of that arising from testing in moist air . The embrittlement was attributed to hydrogen absorption, and recovery was thought to be due to loss of hydrogen (particularly under vacuum) or to diffusion to traps. Potentiostatic tests revealed... 

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