Page 758 - Corrosion Engineering Principles and Practice

708 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 709

15.4.9 Corrosion in Liquid Metals
Corrosion in liquid metals is applicable to metals and alloy processing,
metals production, liquid metal coolants in nuclear and solar power
generation, other nuclear breeding applications, heat sinks in
automotive and aircraft valves, and brazing operations. Corrosion
damage to containment materials is usually the concern. Again,
practical design and performance data are extremely limited. Several
possible corrosion mechanisms need to be considered during the
materials selection design phase. The most severe problems arise at
high temperatures and aggressive melts. Molten steel is typically
regarded as a nonaggressive melt, whereas molten lithium is much
more corrosive.
Practical problems are complicated by the fact that several of these
forms can occur simultaneously. In fact, opposing actions may be
required for individual effects that act in combination. The following
categories can be used to classify relevant corrosion phenomena [16]:

• Dissolution
• Impurities and interstitial reactions
• Alloying
• Compound reduction

Corrosion reactions can occur by a simple dissolution mechanism,
whereby the containment material dissolves in the melt without any
impurity effects. Material dissolved in a hot zone may be redeposited
in a colder area, possibly compounding the corrosion problem by
additional plugging and blockages where deposition has taken place.
Dissolution damage may be of a localized nature, for example, by
selective dealloying. The second corrosion mechanism is one of
reactions involving interstitial (or impurity) elements such as carbon
or oxygen in the melt or containment material. Two further subforms
are corrosion product formation and elemental transfer. In the former
the liquid metal is directly involved in corrosion product formation.
In the latter the liquid metal does not react directly with the
containment alloy; rather, interstitial elements are transferred to,
from, or across the liquid.
Alloying refers to the formation of reaction products on the
containment material, when atoms other than impurities or interstitials
of the liquid metal and containment material react. This effect can
sometimes be used to produce a corrosion-resistant layer, separating
the liquid metal from the containment (for example, aluminum added
to molten lithium contained by steel). Lastly, liquid metal can attack
ceramics by reduction reactions. Removal of the nonmetallic element
from such solids by the melt will clearly destroy their structural
integrity. Molten lithium poses a high risk for reducing ceramic
materials (oxides).