The Origin of Alloy 20

The origin of Alloy 20 can be traced back to the invention by Carpenter Steel in 1951. The alloy was introduced as Carpenter Stainless 20 and represented the development of a wrought alloy that was similar to the cast alloy CN7M. The cast alloy was developed by DuPont to satisfy the need for a material resistant to sulfuric acid. The cast composition could not be hot worked, however, and Carpenter’s invention made the wrought product forms (plate, sheet, bar, wire, pipe, forgings) possible.

Alloy 20 ChartSeveral years later, the alloy was further modified by Carpenter to improve its performance in welded construction by stabilizing the alloy, using columbium (niobium) as an alloying element. This alloy was called Carpenter 20Cb. With additional modifications to enhance its usefulness in a range of severe environments the alloy was modified again with a new patent in 1965 when the alloy was identified as Carpenter 20Cb-3®. The broad chemistry limits of this patent (3,168,397) established the chemistry limits for the current “Alloy 20” recognized officially as UNS N08020.

The patent has long since expired and the alloy is in the public domain. While there are multiple producers of the broad composition N08020, 20Cb-3 is still a registered trademark of Carpenter Technology Corporation. Carpenter, and those with whom they have shared the technology, produce the alloy to a very tight chemistry range within the broad limits to optimize the alloy’s performance.

The corrosion resistance of UNS N08020 in sulfuric acid is remarkable in that it is one of few materials that can withstand any concentration of sulfuric acid up to about 140° F (60° C) while exhibiting a corrosion rate below 5 mils per year (0.13 mm/yr). Sulfuric acid may be the most widely used industrial chemical with applications in water treatment, chemical and petrochemical processing and the steel industry, to name a few. While it can be handled in mild steel in concentrated form (>95%), it becomes more corrosive upon dilution, also evolving heat, which further increases corrosivity.

Alloy 20 is also useful in other severe acid and caustic environments. Because of the high nickel content, Alloy 20 is also virtually immune to chloride stress corrosion cracking (SCC). This corrosion mechanism is the Achilles heel of the workhorse grades of stainless steel, 304 and 316. Until the mid 1980’s, Alloy 20 was often chosen as lower cost solution to SCC where the only other alternatives were higher nickel alloys such as Alloy C276 or Alloy 625. While it is still used in some areas for its resistance to SCC, a range of other alloys were developed that are more cost effective in many cases. These alloys include duplex alloy 2205, superduplex alloy ZERON 100® and the superaustenitic grade, AL-6XN® Alloy. Information about these, and many other alloys, is available elsewhere on this website.