ISO 27667:2011 – Road Vehicles – Brake Lining Corrosion Effects on Painted Backing Plates and Brake Shoes

1. Overview of ISO 27667

ISO 27667:2011, developed by ISO/TC 22 (Road vehicles, Subcommittee SC 2, Braking systems and equipment), defines standardized test methods for evaluating corrosion effects on painted backing plates used in disc brakes and painted brake shoes used in drum brakes. Corrosion of brake components is a significant concern in the automotive industry, as it can lead to warranty claims, reduced braking performance, and costly repairs over the vehicle lifecycle.

Brake backing plates and brake shoes are underbody components exposed to harsh environmental conditions including road salt, moisture, temperature extremes, and chemical contaminants. A robust protective paint system is essential for ensuring long-term durability throughout the vehicle’s service life.

The standard focuses on phases 5 and 6 (product specification/validation) of ISO 15484. It provides a harmonized set of laboratory test procedures and assessment criteria aligned with ASTM, SAE, and JASO test protocols.

2. Corrosion Test Methods

ISO 27667 specifies two primary test methods for laboratory corrosion evaluation:

Test Method	Reference Standard	Scope	Duration	Key Parameters
Neutral Salt Spray (NSS)	ISO 9227	Accelerated corrosion testing for metallic materials; comparative quality assessment	2 h to 96 h (96 h recommended for disc pads and drum shoes)	35 C +/- 2 C; NaCl 50 g/l +/- 5 g/l; pH 6.5-7.2; fog 1-2 ml/h per 80 cm2
Chemical Resistance	ISO 2812-1	Determination of paint and varnish resistance to liquids	48 h immersion in each liquid	Ambient temperature; vertical immersion; samples 30 mm apart

2.1 Neutral Salt Spray Testing (ISO 9227)

The NSS test exposes painted brake components to a controlled corrosive atmosphere of atomized salt solution at 35 C. The standard emphasizes that NSS test results should be regarded as a comparative quality check, not a direct guide to in-service corrosion resistance.

Artificial aging tests, including salt spray, may not replicate the exact conditions experienced in actual vehicle operation. Field testing should be conducted whenever feasible to validate laboratory findings.

2.2 Chemical Resistance Testing (ISO 2812-1)

This method evaluates the paint system’s resistance to chemicals. The standard specifies immersion testing in lubricating oil (48 hours) and brake fluid (48 hours). Three methods are available: immersion, absorbent disc, and spotting.

3. Evaluation and Assessment Criteria

After corrosion testing, paint system degradation is evaluated using a comprehensive set of ISO standards:

Adhesion (ISO 2409): Cross-cut test to measure coating adhesion to the substrate.
Blistering (ISO 4628-2): Assessment of quantity and size of blisters.
Rusting (ISO 4628-3): Assessment of the degree of rusting.
Cracking (ISO 4628-4): Assessment of the degree of cracking.
General defect designation (ISO 4628-1): Overall rating system for defects.

The multi-standard evaluation approach recognizes that corrosion manifests in multiple failure modes: adhesion loss, blistering, rusting, and cracking.

4. Engineering Design Insights

Sampling strategy matters. The standard requires test samples to have been through the full production process per ISO 15484. Corrosion resistance depends on paint formulation, surface preparation, application method, curing, and handling.

Control and reference samples are mandatory. Control samples validate test equipment, while reference samples provide a baseline for comparison.

The standard acknowledges the limits of accelerated testing. No widely accepted models exist for predicting in-service life based on laboratory corrosion testing alone.

In brake systems, heat generated during braking can keep the micro-environment temperature at the backing plate well above ambient. This thermal effect can reduce time of wetness but may accelerate chemical corrosion reactions. Engineers must consider this thermal-corrosion interaction.

5. Frequently Asked Questions

Q1: Can ISO 27667 testing fully predict real-world corrosion?
No. Artificial aging tests may not have the same effect as natural exposure. Field testing is recommended for final validation.

Q2: Why distinguish specification/validation from production monitoring?
Specification/validation requires thorough analysis. Production monitoring needs simple, reliable checks to verify process conditions.

Q3: What chemicals are covered?
Lubricating oil and brake fluid. Additional chemicals may be tested upon agreement.

Q4: How does paint thickness affect corrosion protection?
A trade-off exists between corrosion protection (thicker coating) and heat transfer efficiency (thinner coating).