Alloy 28 was originally developed for wet-process phosphoric acid and is today the most widely used metallic material for evaporator tubes in that duty, with units in service for over ten years where graphite heat exchangers previously suffered repeated tube failures. Laboratory testing in wet-process phosphoric acid shows it to be far more resistant to the chloride and free fluoride impurities derived from phosphate rock than other high-alloy materials. In sulphuric acid it suits piping and heat exchangers particularly between 40 and 70% deaerated acid and above 85%, with approximately the same resistance as the nickel-base C-type alloys in concentrated 98% acid. It is used for production tubing, casing and liners in deep sour gas wells under NACE MR0175, for fluoride-bearing off-gas handling, for nuclear plant heat exchangers on the strength of its stress-corrosion cracking resistance, and for seawater and chloride-bearing cooling water service.
One honest limit: the manufacturer's own datasheet states that super duplex SAF 2507 is more resistant than Alloy 28 in seawater, so for a pure seawater duty a super duplex grade may be the better and cheaper answer; Alloy 28 earns its place where seawater is combined with an aggressive cooled medium, or where acid resistance governs.
The S alloy numbers are brand designations, not UNS numbers, and they do not map onto the alloy numbers used elsewhere in this catalogue. Alloy 41 in this family is UNS N08825 — the alloy sold generically as 825 — not the Rene 41 superalloy. Alloy 60 is UNS N06625, which is alloy 625, while Alloy 70 is UNS N06600, which is alloy 600. Alloy 31HT is UNS N08810/N08811, the 800H and 800HT chemistry. Always confirm the UNS number rather than ordering against the brand number alone; the
S Alloys cross-reference table sets out the full mapping.
Sanicro is a trademark of Alleima, the former Sandvik Materials Technology business. Exotic Alloy is an independent manufacturer and is not affiliated with or endorsed by Alleima or Sandvik; the designation is used here solely to identify the alloy chemistry, which is defined by the UNS and EN numbers given above.
