J. Zhang and N. Li, Oxidation of Metals 63, pp. 353-381 (2005), “Oxidation mechanism of steels in liquid-lead alloys”
notes by JWF
last updated Feb. 12, 2006
This is the best overview of oxidation and corrosion in this system.
Corrosion in lead-alloy systems arises from high solubility of Fe, Ni, and Cr in the lead-alloy. Corrosion rate depends on flow velocity, temperature and its profile, and composition of the liquid and solid material. If nothing is done, dissolution of the steel is rapid. The Russians have had success in maintaining a protective oxide layer on the surface, separating the lead from the steel. Key to the Russian success was active control of the oxygen level in LBE, in order to maintain an oxide layer on the steel surface. The oxide layer serves as a solid-state diffusion barrier between the steel and the liquid-lead alloy. The ideal case would be an oxide layer that is free of pores or cracks, stress-free at the operating temperature, and resistant to spalling and damage during heating and cooling. In addition, in an ideal case oxygen and metal ions should have low diffusion rates through the oxide. In an ideal case, the corrosion rate would be low enough for a desired service life. While the ideal case is not achievable, it is possible to optimize the system in order to approach the ideal, by controlling the oxygen level,and changing steel composition and operating conditions.
According to Zhang and Li, there is no standard model of oxidation/corrosion of steel in flowing liquid lead or lead-bismuth. Experimental efforts are underway in a number of countries, but so far the data are so scarce that a basic understanding is incomplete. The knowledge base required for design is now not adequate. Instead, Zhang and Li review oxidation in aqueous environments, including the removal of scale, and propose a model for corrosion in LBE.
For martensitic steels, there are two possible structures for the oxide layer:
At temperatures below 550 C, the oxide layer is duplex: i.e., it has two layers: an outer magnetite (Fe3O4) layer, and an inner layer of Fe-Cr spinel-oxide. In some cases the outer magnetite layer is not observed.
At temperatures about 550 C, an internal-oxidation zone with oxide precipitates along the grain boundaries is observed below the Fe-Cr spinel layer.
For austenitic steels (which have more Cr and Ni than ferritic steels), there are three possible results:
At temperatures below 500, the oxide layer is very thin and is composed of a single layer FeCr spinel, which can prevent direct dissolution.
For temperature around 550 C, the oxide layer can have either duplex- or single-layer structure, depending on the surface and operating conditions. The duplex-oxide layer can prevent dissolution of steel components, while the single-layer oxide does not protect.
For temperature above 550 C, heavy dissolution occurs.
A number of corrosion tests have been performed in static liquid lead. In those cases, if the concentration of steel components that are dissolved in the lead reaches the saturation limit, then no further dissolution will occur.
In the case of flowing tests with oxygen-controlled liquid lead, Zhang and Li summarize the possibile oxide structure in four figures (Fig. 1): single layer (of Fe-Cr spinel), single-layer with selective oxidation, duplex-layer, and no oxide layer.
When a piece of steel is exposed to an oxidizing environment, if the steel lacks an oxide layer, it will rapidly form an oxide layer. After the oxide layer reaches 2-3 nm, the growth of the oxide layer slows down, and is controlled by diffusion: either diffusion of the metal outward, or diffusion of the oxidant (oxygen or water) inward.
In a flowing lead system, the lead can move metal (by dissolution) at the outer surface. If this removal rate is high enough, no new oxide can form at the outer surface. Instead, oxide forms at the inner surface, between the oxide layer and the metal. Fe has a higher diffusion rate than Cr and Ni, and consequently Fe diffuses outward to the oxide/liquid interface, and is removed. New oxide takes up the space left by Fe diffusion. In this case of “selective oxide layer growth”, the controlling factor is Fe diffusion.
At higher temperatures, the diffusion rate of Fe increases, and an increasing amount of Fe reaching the interface between the oxide and the liquid lead. When this exceeds the rate at which Fe can be dissolved in the liquid, the excess Fe will be oxidized, and the oxide layer will grow at the liquid/oxide interface. This produces a duplex oxide layer. The interface between the inner and outer layer corresponds to the original steel surface.
The mechanism of duplex-oxide formation is complicated and not well understood, not even in gas-phase oxidation. The basic picture is the following: Fe diffuses outward, producing an iron oxide layer a the outer (liquid/oxide) interface. Since Cr diffuses more slowly, this results in Cr enrichment in a spinel layer near the inner (oxide/base metal) interface. Also, oxygen diffuses inward to produce the Cr-containing spinel layer. The transport of oxygen by pure diffusion is too slow to account for the observed oxidation rates, so there must be other paths for oxygen transport.
If the oxygen concentration is too low to form a protective oxide layer, heavy dissolution occurs. At high temperatures, heavy dissolution can occur even in the presence of an oxide layer, if the oxide layer is not protective.
The oxide layer structure depends heavily on the steel composition (e.g., contents of Cr and Si).