The delamination process of the dross build-up structure on submerged hardware in Zn-Al and Zn-Mg-Al baths

Abstract

Hot-dip galvanizing is a well-known process to increase the corrosion resistance of steel. As a by-product dross is formed in the Zn-bath. The dross particles are composed of Fe, Al and Zn in the form of Fe2Al5Znx and interact with the hardware that is submerged in the Zn-bath and eventually accumulate on the surface of the hardware. This accumulation of dross on the hardware is known as dross build-up.

Dross is formed in the Zn-bath as a result of the dissolution of Fe from the steel strip. This Fe reacts with Al present in the liquid Zn forming the Fe2Al5Znx dross particles. Once the hardware is submerged in the liquid Zn a thin compact Fe2Al5-layer is formed on top of the surface of the hardware, known as the diffusion layer. This diffusion layer acts as a barrier for Fe towards the Zn-bath and Zn towards the hardware surface. Once the diffusion layer is formed dross particles precipitate on top of this diffusion layer followed by a slow growth of the intermetallic dross particles. The diffusion layer and accumulated intermetallic dross particles are known as the dross build-up. The thickness of this dross build-up depends on immersion time, bath temperature and bath chemistry and varies typically between 90 μm up to several millimetres.
The dross build-up is thought to be a critical factor in the bath hardware lifetime. The surface quality of the bath hardware directly influences the quality of products produced on the galvanizing line. More dross in the Zn-bath could lead to more defects in the coating, which could lead to failure when the steel strips are processed further, or giving the coating on the steel strip a bad appearance. Because of the dross build-up on the hardware, the bath hardware is changed every 4-6 weeks. By controlling the dross in the Zn-bath, the service lifetime of the hardware could possibly be extended. When the service lifetime of the hardware is extended the maintenance downtime and costs of the galvanizing lines are reduced.

Recently, Tata Steel introduced a new type of Zn-coating: MagiZinc (MZ). This type of Zn-coating differs from the conventional Zn-coating: Conventional Zn-coating (GI) consists of Zn with 0,30 wt% Al whereas MagiZinc consists of Zn with 1,60 wt% Al and 1,60 wt% Mg. This difference in composition has an influence on the dross build-up formed on the hardware. At Tata Steel, there is some evidence that the dross build-up layer on the bath hardware created in the conventional Zn-bath is diminishing when the hardware is submerged in the MagiZinc-bath.
This thesis project aims to identify the characteristics of this cleaning behaviour in MagiZinc of the dross build-up that is formed on bath hardware when submerged in conventional Zn.

Based on the results obtained from experiments in this study it can be concluded that the delamination process of the intermetallic dross particles is a combination of intergranular diffusion of Zn and crack formation as a result of thermal shock.

By changing the baths from conventional Zn to MagiZinc the composition of the bath changes. As a result the thermodynamic stability of the intermetallic Fe2Al5Znx dross particles change with respect to the liquid Zn-phase in such a way that the intermetallic dross particles partly dissolve in the liquid MagiZinc. As a result, the intermetallic dross particle/liquid Zn interface changes from a faceted to a curved interface. As a consequence of this change in structure intergranular diffusion can take place between the intermetallic dross particles. By the intergranular diffusion of Zn the cohesion of the grain boundaries of the intermetallic dross particles is reduced. This reduction in cohesion is probably the start of the delamination of the intermetallic dross particles, breaking into smaller pieces at the grain boundaries when enriched with Zn.
The diffusion layer remains largely unaffected in the delamination process due to the better adhesion to the hardware surface compared with the adhesion of the intermetallic dross particles to the diffusion layer. The better adhesion of the diffusion layer to the hardware surface is the result of diffusion of Cr and Ni from the 316L SS substrate into the liquid Zn at time of immersion. The area from where Cr and Ni are dissolved, Al is diffusing into the 316L SS substrate forming the diffusion layer. Simultaneously the intermetallic dross particles form at the hardware surface. This layer is mainly formed from Fe and Al from the bath, not from the 316L SS substrate.
Due to this limited bonding between the intermetallic dross particles and the diffusion layer the intermetallic dross particles are more prone to crack formation due to thermal shock.

By changing the Zn-bath from conventional Zn to MagiZinc and vice versa the hardware and thus also the dross build-up rapidly cools down to room temperature. This thermal shock creates stresses within the intermetallic dross particle-layer. Due to a difference in thermal expansion between the 316L stainless steel matrix and the intermetallic dross particle-layer large cracks form in the intermetallic dross particle-layer. When the hardware is immersed again in liquid Zn the cracks still exist and these cracks accelerate the delamination process of the dross build-up structure. This observed mechanism is independent from the type of Zn-bath that the hardware is immersed in. The process also takes place when the hardware is taken out a conventional Zn-bath and placed back into a conventional Zn-bath.