Materials of Construction - Metals
December 15, 2023

Introduction

All metals corrode to some degree in contact with sulphuric acid.  The challenge is to select a material that will corrode the least.  The degree of corrosion depends on the many factors such as:

• Acid Concentration
• Temperature
• Velocity
• Impurities (i.e. chlorides, fluorides, metals)
• Aeration

The following are only some of the materials that can be used in sulphuric acid plant service.

Ordinary steels if not protected from the atmosphere will oxidize or rust.  Stainless steels also rust but instead of the unsightly and destructive iron oxide film being formed an invisible and highly protective chrome oxide film forms on the surface.  It is the presence of chromium in amounts exceeding 11.5% that permit this protective film to form an give stainless steel its exceptional corrosion resistance properties.

If the protective film is disturbed, it will renew itself if oxygen is present.   An acid solution would corrode stainless steel if there was no oxygen present in the solution.  Simply aerating the solution provides the oxygen required for the chrome oxide layer to form and the material becomes corrosion resistant to the solution.

Nickel is also present in stainless steels and enhances the corrosion properties of the alloy when its concentration is over 7%.   Nickel also improves the ductility of the material.

Molybdenum in stainless steels improves resistance to pitting and crevice corrosion.  In combination with chromium, molybdenum improves the material's resistance to oxidizing agents.  When the chromium level is aboe 18% wt., molybdenum is three times more effective at enhancing the material's abiltity to resist pitting and oxidation than chromium alone.  Pitting resistance increase as the amount of molybdenum increases. However, the material's resistance to sulphuric acid decreases.

The presence of copper in superaustenitic stainless steels provides general corrosion resistance in sulphuric acid.  Copper stabilizes the passive film on the surface of the steel.