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Acids & other Aggressive Chemicals

Common names for chemicals and selection of appropriate stainless steel grades

The table is referenced by the common name. The 'appropriate grades' notes are taken from the Avesta Sheffield Corrosion Handbook and are shown for general guidance only. Specific grade selections should be rechecked with corrosion tables. A link to the BSSA web site article that describes where to find the corrosion tables on the Outokumpu Stainless web site can be found in the Related Links section of this article.

Selection of stainless steels for handling citric acid (C3H4OH(COOH)3)

Citric acid is a weak organic acid, found in fruits such as lemons (citrus) Either the 304 or 316 types can be considered for most storage and handling applications. Citric acid is also be used for cleaning and passivating stainless steels.

Selection of stainless steels for handling acetic acid (CH3COOH)

Acetic acid is a weak reducing acid. It is used in plastics manufacture and is a constituent of foods as vinegar. Ferritic stainless steels such as 430 type can be considered but normally the 304 types are used for most applications, including handling and storage.

Selection of stainless steels for handling nitric acid (HNO3)

Nitric acid is strongly oxidising and promotes the resistance of stainless steel to corrosion. Generally stainless steels are resistant to corrosion in nitric acid. Nitric acid is used in the chemical 'passivation' of stainless steels.

Selection of stainless steels for handling sulphuric acid (H2SO4)

Sulphuric acid is oxidising when concentrated but is reducing at low and 'intermediate' concentrations. The response of most stainless steel types is that in general they are resistant at either low or high concentrations, but are attacked at intermediate concentrations.

Selection of stainless steels for handling phosphoric acid (H3PO4)

Phosphoric Acid is also know as orthophosphoric acid and is classed as a weak acid. Austenitic stainless steels have good corrosion resistance to chemically pure phosphoric acid. Wet process phosphoric acid (WPA) can be aggressive.

Selection of stainless steels for handling hydrofluoric acid (HF)

Hydrofluoric Acid is extremely aggressive and attacks most metals and glass. The approach to the selection of stainless steels is similar to that for hydrochloric acid.

Selection of stainless steels for handling hydrochloric acid (HCl)

Hydrochloric acid is classed as reducing acid and lacks the oxidising properties that stainless steels need to maintain their 'passive' corrosion resistant surface layer.

Selection of stainless steels for handling sulphur dioxide (SO2) and sulphur trioxide (SO3)

Sulphur dioxide dissolves readily in water, which is then classed as a weak reducing acid (sulphurous acid H2SO3). The oxidation of sulphur dioxide can be assisted by chlorine. Sulphuric and hydrochloric acids can be formed in aqueous (water) systems.

Selection of stainless steels for handling ammonia (NH3)

Ammonia at normal atmospheric temperatures and pressures is a gas. It can be stored under pressure as a liquid or below it's atmospheric boiling point of -34 degC.

Selection of stainless steels for handling sodium hypochlorite (NaOCl)

Sodium hypochlorite only exists in solutions. The solution can be unstable, giving off chlorine gas. Sodium hypochlorite is not stable as a solid chemical.

Selection of stainless steels for handling chlorine (Cl2) and chlorine dioxide (ClO2)

Chlorine is a powerful oxidising agent. It normally occurs as a gas but can be liquefied under pressure at ambient temperatures. As a dry gas it does not tend to be aggressive towards stainless steels, but as either a damp gas or dissolved in water it can be a corrosion hazard.

Selection of stainless steels for handling sodium hydroxide (NaOH)

Sodium Hydroxide (Caustic Soda) is a strong base. It is used in metal degreasing and cleaning processes in a wide range of industry applications.

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