- Published on Wednesday, 22 April 2009 22:00
- Written by Thomas Spence
Q: What do I need to know to choose the right fasteners for my valves?
A: Choosing the proper fastener alloy is one of the most important considerations for reliable long-term operation of valves and other chemical process equipment. The easy part is choosing a fastener with adequate strength whereas selecting fasteners with the necessary corrosion resistance is somewhat more difficult. Choosing fasteners for their general corrosion resistance is certainly an obvious consideration, but it is sometimes even more important to use and specify fastener materials that will be resistant to the various forms of environmental stress corrosion cracking (SCC). Therefore, in certain environments it may be necessary to choose a bolt material that will experience some general corrosion but will provide adequate resistance to stress corrosion cracking. The logic behind this practice is that general corrosion is visually evident and with proper preventative maintenance, general corrosion can be seen and fasteners replaced as necessary. On the other hand, stress corrosion cracking is rarely visually apparent and often occurs without warning. When it does, then a serious valve failure is likely.
There are several forms of stress corrosion cracking that we need to be aware of such as chloride SCC, hydrogen sulfide HSSC, caustic embrittlement and liquid metal embrittlement. Because austenitic stainless-steel fasteners like ASTM A193 grade B8 provide good general corrosion resistance, they are commonly requested. However, one must exercise care as to their use if there is reason to believe that chloride SCC may be an issue. Chlorides as well as other halogens such as fluorides may be present in many CPI plants, and they can be the catalyst for SCC of austenitic stainless steels like 304 or 316. Chlorides are also present in the environment of seacoast plants. Because marine environments are very corrosive to carbon steels, many plants request stainless-steel fasteners but this may lead to the potential for a serious failure due to chloride SCC. Therefore, for marine environments or other chloride-containing services, alloy steel fasteners are preferred. In order to reduce their susceptibility to general corrosion, alloy steel fasteners like grade B7 are usually provided with some type of protective coating such as zinc or cadmium plating. Unfortunately, this can lead to another form of environmental stress cracking known as liquid metal embrittlement (LME), or a related failure mode, solid metal induced embrittlement (SMIE).
Zinc and cadmium plating are commonly used to provide galvanic protection to carbon steel fasteners so they will have an acceptable general corrosion rate. This is a useful practice for moderate temperatures but at elevated temperatures either LME or SMIE may occur. LME is the brittle failure of a normally ductile metal when in contact with a thin film of liquid metal and stressed in tension. The source of the thin film of liquid metal is the cadmium or zinc used for galvanic protection of steel from general corrosion. Cadmium and zinc have relatively low melting points so when plated fasteners are used near or above their melting point LME is possible. SMIE is similar, but occurs below the melting point of the embrittling metal. Prevention of LME and SMIE can be accomplished by avoiding plated fasteners above certain maximum temperatures. According to the available literature, this temperature is 370º F (188º C) for cadmium and 489º F (254º C) for zinc-plated alloy steel. Due to other environmental concerns, cadmium is not used much today but it is still available so you need to be sure of what type of plated fasteners you are getting from your supplier. I know of one case where zinc-plated fasteners were ordered but the supplier substituted some cadmium-plated fasteners by mistake. Since the service temperature was above 370° F (188° C), an LME/SMIE failure resulted.
Another common concern about fastener selection is hydrogen sulfide stress cracking, HSSC. Most metals are susceptible to stress cracking if exposed to hydrogen sulfide, and fasteners that are high strength and highly stressed are very susceptible to HSSC. Therefore, the NACE specifications, MR0103 and ISO 15156, which deal with hydrogen sulfide services, state that if fasteners are exposed to hydrogen sulfide they need to be made of certain alloys. For MR0103 those are ASTM A193 grades B7M or B8MA class 1A and A320 grade L7M. For ISO 15156 the recommend fasteners are A193 grade B7M or A320 grade L7M.
Some other services that can cause SCC with certain fasteners are caustic compounds and hydrofluoric acid. Fortunately, most atmospheric environments do not contain the stress cracking agents discussed here but if there is any chance of exposure, give careful consideration to the selection of fasteners. Choosing the right fastener is rather complex so use a fastener supplier that understands these issues or consult with a corrosion engineer.