Picking corrosion resistant metals for a chemical plant, one fluid at a time

Most of a chemical plant is not exotic. Cooling water, process water, alcohols, ketones, many organic acids at moderate temperature and nitric acid are handled well by 304L and 316L. The L grades keep carbon at 0.030% max, which stops chromium carbides forming at weld heat-affected zones. That matters because almost every tank and pipe in a chemical plant is welded.

Choose 316L stainless where any chlorides are present, even at low levels. The 2–3% molybdenum is what holds off pitting.

Chlorides are the usual killer of stainless steel. Above about 60 °C, 304 and 316 can suffer stress corrosion cracking, and stagnant zones under deposits pit through. Duplex steel is the next step up: higher chromium, molybdenum and nitrogen give it a much higher pitting resistance number and good resistance to chloride cracking. Brine, seawater coolers and many effluent lines use it.

Duplex also has roughly twice the yield strength of 316L, so a designer can often reduce wall thickness and claw back part of the price difference. The catch is welding: duplex needs controlled heat input and matching filler to keep the ferrite balance right, so make sure your fabricator has a qualified procedure before you switch.

Ordinary stainless steel has almost no life in hydrochloric acid. Hastelloy C-276 and related nickel-molybdenum-chromium alloys are the standard answer for reactors, agitators, valves and heat exchangers handling HCl, wet chlorine, hypochlorite and mixed acid streams with oxidising impurities. They cost a lot more, so plants use them only on the wetted parts that need them.

Lead forms a tight lead-sulphate film in sulphuric acid and has been used for acid tank linings, pickling baths and acid plant parts for over a century. It is soft and heavy, so it is usually a lining over steel rather than a structure. Incoloy 825 is the modern alloy choice for many sulphuric and phosphoric acid heat exchangers and pipes.

Where you are replacing an existing lined tank, measure the old lining thickness and tell us the area to be covered. Lead is sold by weight, so the quote depends on it.