Liquid (molten) metal corrosion resistance of stainless steels

Introduction

Most of the literature on liquid metal corrosion of stainless steels discusses metals such as sodium, potassium, lithium etc. which have been used as liquid metal coolant media in applications such as nuclear power plant.
The metals that are of more interest to most industrial users are probably aluminium, copper, lead, tin and zinc for either electric circuit board soldering equipment or in non-ferrous foundry applications.
Most of these types of applications will involve short-term contact, whereas corrosion table data generally considers long term contact. Some interpretation of the information available is therefore generally needed.

Attack can be complex, depending on : -

 

  • Composition and impurity levels

 

 

  • Temperature, heating and cooling cycles and times

 

 

  • Liquid metal velocity (turbulence)

 

The attack can be uniform (transgranular) or intergranular, which makes using corrosion table data difficult.

Resistance to molten aluminium (m.p. 660°C)

Aluminium should be considered as aggressive, stainless steels having limited corrosion resistance at the melting point and poor long-term corrosion resistance at 700°C. Some foundry operators have suggested acceptable performance from ladle parts and stirrers; others have found service life to be erratic and corrosion to be a problem.

Resistance to molten copper (m.p. 1083°C)

There is little information available, but grain boundary attack of stainless steels can be expected. The ferritic types such as 1.4016(430) may be better choices than austenitics, which can be expected to fail rapidly in molten copper and copper alloys.

Resistance to molten lead (m.p. 327°C)

Lead, in the presence of oxygen, is more aggressive than lead covered with a charcoal layer. At 400°C some attack to 1.4301 (304) type can be expected. The 1.4401(316) type and higher alloyed austenitics should be resistant.
If temperatures are as high as 900°C then a charcoal oxidation-inhibiting layer is essential if severe corrosion is to be avoided. Any antimony in the lead can increase the risk of corrosion.

Resistance to molten zinc (m.p. 419°C)

Corrosion table data indicates that zinc at 500°C is very aggressive to ferritic and austenitic stainless steels, but for short tem contact, type 316 has been noted as being worth considering. Intergranular attack can be a hazard however with molten zinc.
The higher alloyed 6% molybdenum austenitic types, such as 1.4547 (254SMO), have been shown to be significantly better than 1.4401(316) type and are worth considering, if the extra cost can be justified.

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