Corrosion types hydrogen embrittlement

Tantalum-Titanium Bishop examined the corrosion resistance of this alloy system in hydrochloric, sulphuric, phosphoric and oxalic acids and found that alloys containing up to about 50% titanium retained much of the superlative corrosion resistance of tantalum. Under more severe conditions, a titanium content of below 30% appears advisable from the standpoint of both corrosion resistance and hydrogen embrittlement, although contacting or alloying the material with noble metals greatly decreases the latter type of attack. Tantalum-titanium alloys cost less than tantalum because titanium is much cheaper than tantalum, and because the alloys are appreciably lower in density. These alloys are amenable to hot and cold work and appear to have sufficient ductility to allow fabrication. 

The martensitic alloys contain 12 to 20 percent chromium with controlled amounts of carbon and other additives. Type 410 is a typical member of this group. These alloys can be hardened by heat treatment, which can increase tensile strength from 550 to 1380 MPa (80,000 to 200,000 Ibf/in ). Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, freshwater, and organic exposures). In the hardened condition, these materials are very susceptible to hydrogen embrittlement. 

A number of internal components, such as valves, valve seats, cylinder walls, pistons, and rings, will be exposed to hydrogen and water vapor. The potential effects are of two primary types (1) decarburization of steels and cast iron and (2) hydrogen embrittlement of aluminum pistons. Water vapor could cause excessive corrosion of exhaust systems, but this could be minimized by use of titanium. 

Fig. 7.71 Potential ranges of stress-corrosion cracking by (I) hydrogen embrittlement, (II) cracking of unstable passive film, and (III) cracking initiated by pits near the pitting potential. Vertical dashed lines define potential range over which nonpassivating type films may crack under stress.

For each aTToy, experiments Have been done in which specimens were exposed unstressed, to solutions that can cause cracking, under a variety of conditions. If broken in air immediately after removal from the solution, specimens exhibited a low value of Zf and fracture surfaces characteristic of stress corrosion cracking. If lapse of time occurs between removal from solution and stressing, specimens exhibited increased values of Sf and decreased amounts of stress corrosion-type fracture with increasing length of lapse of time. This behavior is characteristic of hydrogen embrittlement fracture and has been interpreted as such for the four types of alloys described. These are simple but very clear experiments. 

In addition to hydrogen embrittlement, brittle failure can occur as a result of stress corrosion, liquid metal attack, or strain-age hardening. The last mentioned cause is well known and can occur on strained steel of any strength but seldom actually occurs in modem steels it is adequately documented in Appendix E of BS 729 (British Standards Institution, 1971 reaffirmed in 1986) users often erroneously refer to this effect as hydrogen embrittlement. This clearly states that strain-age embrittlement is the only type of embrittlement that can be aggravated by the hot dip galvanizing process. 

Titanium is mainly susceptible to two types of localized attack - crevice corrosion and environment-assisted cracking (stress-corrosion cracking and/or hydrogen embrittlement). Titanium is resistant to pitting cor-... 

On average, the general corrosion resistance is below that of t) e 304 stainless. However, the corrosion resistance of t) e PH 15-7 Mo alloy approaches that of type 316 stainless. The martensitic and semiaustenitic grades are resistant to chloride stress cracking. These materials are susceptible to hydrogen embrittlement. 

RM Rieck, A Atrens, lO Smith, Stress corrosion cracking and hydrogen embrittlement of cold worked AISl type 304 austenitic stainless steel in mode I and mode III, Materials Science and Technology, 1986, 2, 1066-1073. 

Screw anchors (Figure 4.4) transfer tension loads via the interlock of the screw threads with matching female threads cut into the concrete by the hardened forward threads. They are often used for light and medium-duty applications where speed and ease of installation are a factor. The high hardness required for cutting the threads into the concrete makes screw anchors susceptible to hydrogen embrittlement and stress corrosion, particularly under the head, and caution should be exercised where they are used in unprotected environments. Depending on the depth of the threads, screw anchors may have superior tension resistance relative to other expansion anchor types in cracked concrete conditions. 

Titanium can also become embrittled by absorbed hydrogen as a result of corrosion or exposure to dry hydrogen gas. When hydrogen is absorbed by titanium in excess of about 150 ppm, a brittle titanium hydride phase will precipitate out. This type of embrittlement is usually permanent and can be reversed only by vacuum annealing, which is difficult to perform. 

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