Ask the Expert: Featuring Doctor Metals
Dr. Hira S. Ahluwalia is president of Material Selection Resources (MSR), Inc., a materials engineering and corrosion consulting organization based in New Jersey that serves the chemical, pharmaceutical, fabrication, and metal industries. Dr. Ahluwalia has extensive experience in the field of corrosion and material science, and has published numerous technical papers. His areas of expertise include CUI prevention, material selection, corrosion testing, metallurgy, and failure analysis. He has a PhD in materials and corrosion engineering from Newcastle University in the United Kingdom. A 20-year ASM and NACE member, Ahluwalia is chairman of the NACE Annual Conference Program Committee. He is also a Nickel Institute consultant and Materials Technology Institute (MTI) consultant. He can be contacted at 609-737-8226, email@example.com, and www.doctormetals.com.
Q: Are there specific locations that are more susceptible to corrosion under insulation (CUI) for piping systems?
A: Our experience has shown that penetrations or breaches in the insulation jacketing systems–such as pipe hangers and other supports, dead legs, valves and fittings, bolt-on pipe shoes, and steam and electric tracer tubing penetrations–are more susceptible CUI locations. Low points in piping systems that are known to have a breach and places where water can easily accumulate are also susceptible to CUI. Other areas of concern include termination of insulation at flanges and other piping components; damaged or missing insulation jacketing; insulation jacketing seams located on the top of horizontal piping or improperly lapped or sealed insulation jacketing; termination of insulation in a vertical pipe; and locations where caulking has deteriorated or is missing.
Q: You mentioned in your November 2006 article that duplex alloys may be used to reduce environmental stress corrosion cracking (ESCC). What are duplex stainless alloys?
A: The duplex alloys are characterized by their austenite and ferrite microstructure that results in high strength and excellent chloride stress corrosion cracking (CSCC) resistance, localized corrosion resistance, and general corrosion resistance. The established 2205 duplex grade accounts for 80 percent of duplex usage; however, newer duplex grades have been developed for niche applications. Table 1 provides the chemical composition of the common 2205 grade and some of the “lean” duplex grades.
The “lean” duplex grades developed in recent years include LDX 2101 (Outokumpu), SAF 2304 (Sandvik), and AL 2003 (Allegheny Ludlum). These alloys have low molybdenum additions, and they are less prone to sigma phase formation during processing. LDX 2101 uses manganese rather than nickel to obtain the proper ferrite-austenite phase balance. The corrosion resistance of LDX 2101 is considered better than 304L stainless, and in most cases it is comparable to 316L. Because of the alloy’s good CSCC resistance, it is a good candidate to replace 300 series stainless steels where CSCC due to CUI is a problem.
SAF 2304 is essentially a low-molybdenum, more cost-effective version of 2205. This alloy has found applications in the pulp and paper industry where alkaline-sulfide conditions exist in pulping digesters and in other pulping liquors where molybdenum as an alloying addition reduces corrosion resistance. SAF 2304 has similar pitting and crevice corrosion resistance to 316L, but it offers far superior chloride stress corrosion resistance. AL 2003 was designed to exceed the corrosion resistance of type 316 at substantially lower cost than type 2205 duplex. The alloy has similar localized corrosion resistance to 317L, and it offers superior CSCC resistance.
Readers are encouraged to submit their own insulation questions to industry experts by e-mailing firstname.lastname@example.org. Questions can be on any insulation topic. Future topics will include insulation maintenance, acoustics, power generation and energy issues.