A Contribution to the Understanding of the Metallic Coating of Iron Base Alloys JERRY L. ARNOLD, F. CURTIS DUNBAR, AND CHARLES FLINCHUM In the process of metallic coating of iron base alloys, it is necessary for the liquid metal to wet the base metal if good coating adherence is desired. The presence of a thin skirt of a stable oxide, e.g. A1203, formed on the surface of a steel has been shown to be very detri- mental to proper wetting by liquid metals, e.g. A1 or Zn. Evidence has been gathered which shows that during the normal preparation of the surface of a steel alloy for metallic coating, conditions may oftenexist which promote the formation of an external, stable oxide even if the system is reducing with respect to iron. The theory of the transition from internal to external oxidation has been shown to explain the observed metallic coat- ing adherence behavior. It has been discovered that if an alloy has more than the critical content of an alloying element needed to form a stable, external oxide, the use of an "oxi- dizing preheater" can alleviate this problem and result in a good, adherent coating. The purpose of the "oxidizing preheater" is to form a scale on the surface which is pre- dominantly iron oxide with the other stable oxides dispersed in it. After leaving the pre- heater, the strip passes through the reducing portion of the furnace before it enters the liquid metal. In this reducing furnace the iron oxide part of the scale is easily reduced. In this form the surface is readily wetted by the liquid metal and excellent adherence of the coating to the base metal is obtained. THEwetting of steel surfaces by liquid metals is necessary to obtain quality coated products such as aluminum coated steel, Type 1 and Type 2, and zinc coated steel. The literature1 and various patents (U.S. Patents No. 2,110,893 and No. 3,320,085) teach how excellent adherence may be consistently achieved with the continuousA1 or Zn coating of low carbon steel products. By contrast, it has been observed that erratic coating adherence problems are often experienced when coating low carbon steels which are alloyed with even small amounts of strong oxide- forming elements. A good example is the poor ad- herence of metallic coatings on alloy steels with higher A1 or Si contents which are coated under what are normally considered optimum coating conditions. Armco Steel has recently developed an alloy steel (referred to in this paper as 2SR) which, when alumi- nized, possesses oxidation resistance superior to T409 stainless steel (11 wt pct Cr-1/2 wt pct Ti). This alloy, which is covered under U.S. Patents No. 3,905,780 and No. 3,909,250, has a nominal composi- tion of 2 wt pct Cr, 2 wt pct A1, 1 wt pct Si, and 1/2 wt pct Ti. In the early attempts to aluminize this alloy in the laboratory, complete lack of adherence was experienced when normal coating practices were followed. These experiments were conducted on a 4 in. (0.1 m) wide continuous coating line. Figure 1 is a schematic illustration of this line and the ap- proximate strip temperatures for the experiments which will be described in a later section. The heat- ing portion of this line is composed of two basic sec- tions: i) an initial high intensity, gas fired preheater; and ii) a reducing, radiation heated furnace contain- ing a N2-H2 gas mixture. Conventional coating prac- JERRYL. ARNOLD, Senior Research Metallurgist, F. CURTIS DUNBAR, Senior Research Engineer, and CHARLES FLINCHUM, Engineer, are with ArmcoSteel Corporation, Research Center, Middle- town, OH 45043. Manuscript submitted November 23, 1976. tices call for keepingboth of these sections as clean and reducing as possible such that a fresh metallic iron surface will always be produced on low carbon steel. This metallic iron surface appears to be ideal for optimum wetting by the liquid metal and excellent coating adherence is reliably obtained with low car- bon steels. It was subsequently discovered (U.S. Patent No. 3,925,579) that excellent wetting of this new alloy steel surface by liquid AI could also be consistently obtained if the gas fired preheater were operated with controlled amounts of excess air ("oxidizing pre- heater") as versus the normal operation with excess combustibles. This paper proposes a model explaining these observed phenomena and presents supplemental ex- perimental data supporting it. VARIABLE METALLIC COATING ADHERENCE DURING NORMAL OPERATION OF CONTINUOUS COATING LINE A. Model The model which is proposed to explain the vari- able coatability of steels containingstrongoxide forming elements (e.g. A1, Cr, Si, or Ti) is one de- scribing the transition from internal to external oxidation of the alloy steels prior to their entering the liquid metalbath. When an Fe-X alloy is heated in an environment which is reducing to Fe but oxidiz- ing to X, an XOv oxide phase will be formed (v = ratio of oxygen to X atoms in the oxide). The major con- sideration with respect to the steelbeingwetted by the liquid metalthen becomes one of the location of the formation of this stable oxide. If experimental conditions are chosen such that more oxygen can dif- fuse into the steel than X can diffuse to the surface, the oxide will form as finely dispersed, subsurface precipitates in an iron matrix. This process is called internal oxidation. In this case the exterior surface is still predominantly metallic iron and this should METALLURGICAL TRANSACTIONS B VOLUME 8B, SEPTEMBER 1977-399