In the surface treatment of casting metal parts, zinc, nickel, and chromium plating are three of the most commonly used electrochemical plating processes. The core differences lie in the plating characteristics, process principles, and applicable scenarios. Furthermore, the choice must be made in conjunction with the substrate characteristics of the casting (such as cast iron, cast steel, and cast aluminum). The following is a detailed comparison from the core dimensions:

 

I. Core Definitions and Essential Differences in Plating

The basic attributes (composition, structure, thickness) of the three types of plating are the root cause of subsequent performance differences. First, clarify the core differences:

Coating Types:	Core components:	Coating Structure / Thickness	Core Characteristics (Compatibility with Castings)
Zinc Plating	Zinc (Zn)	Single layer / Multi-layer (e.g., color zinc, black zinc), thickness 5-20μm (8-15μm is commonly chosen for castings)	Low cost, sacrificial anode corrosion protection (preferential oxidation to protect the substrate), low hardness (HV100-200)
Nickel Plating	Nickel (Ni) (pure nickel / nickel alloys, such as nickel-phosphorus alloys)	Single layer / Composite layer (base coat + nickel plating), thickness 10-50μm (castings require hole filling/grinding first)	Medium corrosion resistance, medium hardness (pure nickel HV400-600, electroless nickel plating HV800-1000), high surface finish
Chromium Plating	Chromium (Cr) (decorative chrome / hard chrome)	Double layer / Triple layer (copper base + nickel base + chromium layer), hard chromium thickness 5-100μm, decorative chromium only 0.1-0.5μm	Extremely high hardness (HV800-1200), wear-resistant/temperature-resistant, weak corrosion resistance (requires undercoat protection), bright appearance

II. Process Principles and Key Points for Adapting to Castings

Castings (especially investment castings and sand castings) are prone to micropores and high roughness. The process adaptability of the three types of plating differs significantly:

 

1. Zinc Plating: Low-cost corrosion protection, suitable for cast iron/cast steel parts

Process Principle:

Divided into “electro-zinc plating” (acidic/alkaline plating solution, current deposition) and… Hot-dip galvanizing (immersion in molten zinc) is preferred for castings, while electroplating is the preferred choice (hot-dip galvanizing easily leads to casting deformation, and the coating is thicker and less precise).

Details for casting compatibility:

• Cast iron parts require “degreasing → pickling (removing oxide scale) → activation (removing graphite layer)” to avoid poor coating adhesion;
• Cast steel parts require “passivation treatment” (color zinc, blue zinc, black zinc) after galvanizing to form a passivation film (ZnCrO₄), improving corrosion resistance by 5-10 times;
• Disadvantages: The coating is soft and easily scratched, unsuitable for friction/stress areas.

 

2. Nickel plating: Balances corrosion resistance and smoothness, suitable for precision castings.

Process principle:

Divided into “electroplating nickel” (sulfate/sulfamic acid plating solution) and “chemical nickel plating” (current-free, autocatalytic deposition). Chemical nickel plating is preferred for castings (uniform coating, can cover micropores).

Details on casting compatibility:

• For nickel plating of cast aluminum parts, a zinc immersion primer (to form a zinc layer transition) is required to prevent direct reaction between aluminum and nickel, which could cause the plating to peel off.
• Precision investment castings (such as stainless steel castings) can be polished to a mirror finish after nickel plating to meet appearance and sealing requirements.
• Chemical nickel plating contains phosphorus, achieving a “non-porous plating” layer with better corrosion resistance than zinc plating, suitable for humid/mildly corrosive environments.

 

3. Chromium plating: High wear resistance/decorative, requires a protective underlayer.

Process principle:

Divided into “decorative chromium” (thin chromium layer, corrosion protection relies on a nickel underlayer) and “hard chromium” (thick chromium layer, primarily for wear resistance). Castings must be pre-plated with copper + nickel before chromium plating (chromium layers have high porosity, making direct plating prone to rust).

Casting Part Compatibility Details:

• Hard chrome plating on cast steel/cast iron parts requires grinding to Ra≤0.8μm beforehand; otherwise, the plating is prone to cracking (casting surfaces are rough and require pre-machining).
• Hard chrome plating can repair worn castings (such as hydraulic cylinder bodies), and the plating thickness can compensate for dimensional deviations.
• Decorative chrome is only used for exterior parts (such as cast door handles); the plating is thin and not resistant to strong corrosion.

III. Core Performance Comparison (Suitable for Casting Scenarios)

Performance Dimensions	Zinc plating	Nickel plating (electrochemical plating)	Chrome plating (hard chrome / decorative chrome)
Corrosion Resistance	Excellent (Sacrificial anode, stronger after passivation)	Medium (Good corrosion resistance with non-porous plating)	Poor (requires undercoat, only resistant to slight corrosion)
Hardness / Abrasion Resistance	Poor (Easily scratched)	Medium – Excellent (Wear-resistant electroless nickel plating)	Excellent (hard chrome is wear-resistant / impact-resistant)
High Temperature Resistance	Poor (Zinc layer oxidizes above 100℃)	Medium (Stable below 200℃)	Good (chromium layer is stable below 400℃)
Appearance	Ordinary (Matte/Colored)	Excellent (Can be polished to mirror finish)	Excellent (decorative chrome is mirror-like)
Substrate Compatibility	Cast iron/Cast steel (Preferred)	Cast aluminum/stainless steelprecision parts	Cast steel / cast iron load-bearing / wear-resistant parts

IV. Typical Application Scenarios (Castings as an Example)

Zinc Plating:

• Scenario: Cast steel parts for engineering machinery (such as excavator chassis parts), cast iron pipe fittings for construction, and ordinary hardware castings;
• Logic: Low-cost solution to meet outdoor corrosion protection needs; even if the plating is scratched, zinc can still sacrifice itself to protect the substrate (cathode protection).

Nickel Plating:

• Scenario: Precision investment casting of stainless steel valve cores, cast aluminum instrument housings, and food machinery castings;
• Logic: Non-porous plating prevents media penetration; high surface finish meets sealing/hygiene requirements; chemical nickel plating can cover casting micropores.

Chrome Plating:

• Scenarios: Cast hydraulic cylinder bodies (hard chrome), cast automotive wheels (decorative chrome), cast machine tool guideways (hard chrome);
• Logic: Hard chrome’s high hardness resists friction/impact, decorative chrome enhances the appearance, and the underlying nickel/copper ensures corrosion resistance.

 

V. Summary: Choosing the Core Logic

• If only basic corrosion protection and low cost are required, prioritize zinc plating (suitable for cast iron/cast steel parts);
• If corrosion protection + smoothness/precision sealing are required, choose nickel plating (especially chemical nickel plating, suitable for cast aluminum/precision castings);
• If high wear resistance/high hardness/decorative appearance is required, choose chrome plating (requires a matching base coat process, suitable for load-bearing/appearance castings).

Furthermore, the “pre-treatment” (degreasing, pickling, hole filling, grinding) before plating castings directly determines the plating quality and requires targeted treatment based on the substrate and plating type (e.g., activation before zinc plating for cast iron, zinc immersion before nickel plating for cast aluminum).

 

About Ningbo Suijin

Ningbo Suijin Machinery Technology Co., Ltd. is a professional manufacturer specializing in end-to-end casting solutions. With over a decade of experience in the industry, we focus on “customized R&D + high-precision production + full material compatibility” to provide customers in the machinery manufacturing, aerospace, petrochemical, automotive parts, and precision instrument industries with one-stop customized services from product design optimization to finished product delivery.