Overview
ISO 4520:1981 is an international standard that specifies the requirements for chromate conversion coatings applied on electroplated zinc and cadmium coatings. The primary purpose of these coatings is to provide enhanced protection against corrosion. This standard outlines the application methods, classification, and performance criteria for chromate conversion coatings on zinc and cadmium plating, excluding coatings intended solely for color finishes or improved paint adhesion.
Chromate conversion coatings are widely used to retard the formation of corrosion products such as white rust on zinc and cadmium surfaces exposed to corrosive atmospheres. The coatings improve the durability and lifespan of metal components, making them essential in industries where corrosion resistance is critical.
Key Topics
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Scope and Application
ISO 4520 covers chromate conversion coatings on electroplated zinc and cadmium specifically for corrosion protection. It does not include coatings applied merely for decorative colors or paint adhesion enhancements.
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Method of Application
Chromate coating is typically applied by dipping electroplated components into acidic chromate solutions containing hexavalent chromium salts. The final rinse should be done with hot water cautiously to avoid dissolving the hexavalent chromium. Drying is performed at temperatures not exceeding 60°C to prevent cracking of the coating.
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Classification and Types
The standard classifies chromate coatings into two main classes, each with two types that differ in thickness, appearance, and corrosion protection:
- Class 1: Thin, transparent or iridescent films with slight corrosion protection
- Class 2: Thicker, darker finishes with better corrosion resistance
Finishes may be identified by a combination of class and type designations (e.g., Fe/Zn 25 c 1A).
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Corrosion Resistance Requirements
Coated products must meet minimum neutral salt spray resistance times before white corrosion products appear. These timeframes depend on the chromate coating type, with durations ranging from 6 to 96 hours.
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Handling and Testing Considerations
Newly chromated coatings harden over 24 hours and require careful handling during this time. Corrosion and adhesion tests should be deferred until full curing has occurred.
Applications
ISO 4520:1981 is highly relevant in industries requiring durable corrosion protection for zinc and cadmium electroplated components, including:
- Automotive parts exposed to harsh environments
- Electrical and electronic hardware requiring reliable metal finishes
- Fasteners and mechanical components prone to rust and oxidation
- Aerospace components benefiting from lightweight corrosion-resistant coatings
- General industrial applications where metal corrosion resistance extends service life
Use of chromate conversion coatings ensures improved metal surface stability, less frequent maintenance, and better performance in corrosive conditions.
Related Standards
For comprehensive quality and performance management, ISO 4520 should be used alongside related standards including:
- ISO 2081: Metallic coatings - Electroplated coatings of zinc on iron and steel
- ISO 2082: Metallic coatings - Electroplated coatings of cadmium on iron and steel
- ISO 3613: Chromate conversion coatings on zinc and cadmium - Test methods
- ISO 3768: Metallic coatings - Neutral salt spray test (NSS)
- ISO 3892: Conversion coatings on metallic materials - Gravimetric methods for determination of coating mass
These standards collectively govern plating thickness, corrosion test methods, coating mass, and overall quality assurance for metal coatings, supporting reliable protection strategies in industrial applications.
By adhering to ISO 4520:1981, manufacturers and engineers can ensure effective application of chromate conversion coatings that provide robust corrosion resistance on zinc and cadmium electroplates, optimizing product durability and performance in demanding environments.
