Surface engineering — the zinc diffusion coating process applied to steel components is specifically known as which treatment?

Introduction / Context:
Several processes provide corrosion protection to steel through zinc or other surface modifications. Although the outcomes can sometimes seem similar, the underlying mechanisms are distinct. This question tests recognition of the process in which zinc is diffused into the steel surface rather than simply deposited on it as a thick external layer.

Given Data / Assumptions:

• Objective: form a protective zinc-rich surface layer.
• Substrate: steel or iron components.
• Process candidates: galvanising, anodising, parkerising, and sherardizing.

Concept / Approach:

Sherardizing is a zinc diffusion process conducted typically in a rotating, sealed drum where steel parts are heated with zinc dust and inert fillers. At temperature, zinc diffuses into the steel, creating a graded zinc-iron alloy layer with excellent adhesion and uniform coverage, including internal recesses. Galvanising is hot-dip coating in molten zinc (not a diffusion-only process). Anodising is an electrolytic oxidation used on aluminium and some other non-ferrous metals, not on steels for zinc. Parkerising is a phosphate conversion coating that improves paint adhesion and provides limited corrosion resistance, not a zinc diffusion treatment.

Step-by-Step Solution:

Verification / Alternative check:

Surface finishing standards and handbooks consistently define sherardizing as the zinc diffusion process for ferrous components, distinct from hot-dip galvanising and electroplating.

Why Other Options Are Wrong:

Galvanising — Hot-dip deposition, not diffusion-only.
Anodising — Oxide thickening on aluminium; not applicable to steel for zinc layers.
Parkerising — Phosphate conversion coating; does not introduce zinc by diffusion.

Common Pitfalls:

Using the term galvanising as a catch-all for any zinc-related protection; overlooking that sherardizing provides uniform coverage in threads and blind holes due to diffusion.

Final Answer:

sherardizing