Stress corrosion cracking prevention

Parts and components that come in contact with wet hydrogen sulfide should be provided with sulfide stress corrosion cracking prevention measures in accordance with the latest edition of NACE MR 01-75. 

Brasses are susceptible to dezincification in aqueous solutions when they contain >15 wt% zinc. Stress corrosion cracking susceptibiUty is also significant above 15 wt % zinc. Over the years, other elements have been added to the Cu—Zn base alloys to improve corrosion resistance. For example, a small addition of arsenic or phosphoms helps prevent dezincification to make brasses more usefiil in tubing appHcations. 

Conditions that favor dezincification include stagnant solutions, especially acidic ones, high temperatures, and porous scale formation (2). Additions of small amounts of arsenic, antimony, or phosphoms can increase the resistance to dezincification. These elements are, however, not entirely effective in preventing the dezincification of the two-phase (cc—P) brasses because dezincification of the P-phase is not prevented (31). Another area of corrosion concern involves appHed or residual stresses from fabrication that can lead to EIC of brasses in the form of stress-corrosion cracking. 

To reduce or prevent stress corrosion cracking the following methods can be employed ... 

Corrosion fatigue, therefore, is a special case of stress-corrosion cracking and fatigue failure. Figure 4-451 shows an example of pipe failures due to corrosion fatigue. Corrosion fatigue can be prevented or reduced by ... 

As with alloys of other metals, nickel alloys may suffer stress-corrosion cracking in certain corrosive environments, although the number of alloy environment combinations in which nickel alloys have been reported to undergo cracking is relatively small. In addition, intergranular attack due to grain boundary precipitates may be intensified by tensile stress in the metal in certain environments and develop into cracking. Table 4.28 lists the major circumstances in which stress corrosion or stress-assisted corrosion of nickel and its alloys have been recorded in service and also shows the preventive and remedial measures that have been adopted, usually with success, in each case. 

The requirements for stress-corrosion cracking to occur are a susceptible alloy, the presence of stress, an appropriate environment and a particular range of potentials. Cracking can be prevented by altering one or more of these. The general comments made by Hines" are still valid and worth reiterating with some minor supplementation in the light of more recent data. 

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. 

Where there is a perceived risk of crevice corrosion, cathodic protection can often be used to prevent its initiation. Once more a 100 mV cathodic polarisation will usually prove sufficient. However, it is doubtful whether cathodic protection can arrest crevice corrosion once started and, despite claims to the contrary, whether it could be effective in arresting stress-corrosion cracking. The problem lies in the fundamental difficulty of forcing cathodic current into an occluded area. 

Nature of the environment This is usually water, an aqueous solution or a two- (or more) component system in which water is one component. Inhibitors are, however, sometimes required for non-aqueous liquid systems. These include pure organic liquids (Al in chlorinated hydrocarbons) various oils and greases and liquid metals (Mg, Zr and Ti have been added to liquid Bi to prevent mild steel corrosion by the latter ). An unusual case of inhibition is the addition of NO to N2O4 to prevent the stress-corrosion cracking of Ti-6A1-4V fuel tanks when the N2O4 is pressurised... 

It has been reported that hydrogen embrittlement is a form of stress corrosion cracking (SCC). Three basic elements are needed to induce SCC the first element is a susceptible material, the second element is environment, and the third element is stress (applied or residual). For hydrogen embrittlement to occur, the susceptible material is normally higher strength carbon or low-alloy steels, the environment must contain atomic hydrogen, and the stress can be either service stress and/or residual stress from fabrication. If any of the three elements are eliminated, HE cracking is prevented. 

The use of hydroxyacetic/formic acid in the chemical cleaning of utility boilers is common. It is used in boilers containing austenitic steels because its low chloride content prevents possible chloride stress corrosion cracking of the austenitic-type alloys. It has also found extensive use in the cleaning operations for once-through supercritical boilers. Hydroxyacetic/formic acid has chelation properties and a high iron pick-up capability thus it is used on high iron content systems. It is not effective on hardness scales. 

Chloride content is limited in order to prevent stress-corrosion cracking. 

Zirconium is used in nitric acid service for cooler condensers, tail gas preheaters and reboilers. It rivals tantalum in its corrosion resistance to nitric acid at all concentrations up to the boiling point. Its resistance extends up to 230°C and 65 wt %. However it is susceptible to stress corrosion cracking, which can be prevented by avoiding high, sustained tensile stresses104. 

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