GMW15359 White Corrosion Rating Scale Guide

The purpose of the rating scale described in the standard "GMW15359 White Corrosion Rating Scale Guide" developed by General Motors is to ensure comparison between vehicles and consistency among personnel. Corrosion rating involves the detection of an objectionable surface, and judgment is largely subjective. Fastener corrosion scales provide objective criteria to guide subjective judgment. Essentially, nothing in this standard supersedes applicable laws and regulations.

The GMW15359 standard is a General Motors engineering standard that defines how to evaluate and rate the severity of white (white rust) corrosion (usually zinc corrosion) on automotive parts.

This guide provides a standard rating scale for assessing the visual severity of white rust (zinc corrosion) on coated or plated components, ensuring consistency among GM/Opel laboratories and suppliers. It is often used in quality control and corrosion testing procedures following salt spray tests or other environmental exposure methods.

Typical applications of this standard include:

• Zinc-plated steel parts (e.g. fasteners, clips, small steel hardware)
• Components subjected to corrosion testing (e.g. standard GMW15356 for sheet metal or standard GMW15358 for fasteners)
• Along with other corrosion standards such as GMW15282 (backfire) and GMW15356

While full-scale proprietary, such white rust grading guides typically provide:

• Defines multiple grades / different visual levels (e.g. from 0 to 5 or 0 to 10)
• Shows photographic or schematic examples for each corrosion severity
• Provides criteria based on percentage coverage, blistering, or pattern types (e.g., localized or widespread damage)

This rating scale allows raters to:

• Compares samples consistently
• Determines acceptance and rejection thresholds
• Tracks corrosion performance trends throughout test cycles

This standard is often cited in conjunction with other General Motors testing and rating standards:

• GMW15356 Sheet metal grading scale
• GMW15357 Component rating scale
• GMW15358 Fastener rating scale

The GMW15359 standard supports and complements physical corrosion exposure tests (e.g., salt spray, cyclic corrosion).

Essentially, the GMW15359 standard does not define standalone testing procedures such as salt spray or cyclic corrosion; instead, it specifies how to visually assess and grade white corrosion at the end of such tests. In other words, it specifies what to look for and how to score it, not how to create it.

Typical corrosion test methods to which the GMW15359 standard conforms are:

• Salt spray (neutral salt spray) testing: This test is typically performed according to international standards such as GMW3286, ASTM B117, or ISO 9227. After the specified exposure time (e.g., 240 hours for zinc-coated fasteners), samples are inspected using the visual rating scale of the GMW15359 standard. Assessors examine critical surfaces, note the percentage of white rust coverage, and assign a score based on the GMW15359 scale.
• Cyclic corrosion testing: This test is performed using the GMW14872 standard, which simulates wet/dry, salt fog, and humidity cycles. At various cycle intervals (e.g., 5, 10, 20 cycles), the parts are removed, rinsed, dried, and visually graded according to the GMW15359 standard. Photographic documentation and percent area ratings are included (typically from the GMW15356, GMW15357, and GMW15358 standards).
• Scratch rebound analysis: If coatings are intentionally scratched (GMW15282), corrosion rebound from the scratch is measured (in mm) and then visually graded using the GMW15359 standard.

A typical test sequence example is as follows:

• Preconditioning: Parts are cleaned, aged, and sometimes scratched.
• Exposure: Salt spray or cyclic corrosion testing is carried out according to GMW3286 or GMW14872 standards.
• Rinse and dry: Once the cycle or time point is completed, it is rinsed according to standards and then air dried.
• Visual inspection: Under controlled illumination, critical surfaces (e.g. heads, teeth) are inspected using the GMW15359 standard grading scale
• Documentation: Corrosion score is assigned, coverage percentage is recorded and photographs are taken if necessary.

This standard is considered important for the following reasons:

• Ensures consistency in corrosion assessment: This standard provides a clear and repeatable visual rating scale for assessing white corrosion (zinc corrosion) on parts. This prevents subjective judgments and inconsistencies among different laboratories, suppliers, or inspectors. Everyone speaks the same language when describing corrosion severity.
• Supports quality control and product reliability: White rust can compromise the protective zinc coating on metal parts, leading to premature corrosion and potential part failure. Using this standard, manufacturers can reliably monitor corrosion performance during testing and production. It helps determine whether coatings or materials meet durability requirements before parts enter vehicles.
• Assists with supplier management and compliance: Automotive original equipment manufacturers like General Motors rely on suppliers worldwide. The GM15359 standard provides a uniform criterion for inspecting and qualifying the corrosion resistance of suppliers' parts. Suppliers know the exact level of acceptable corrosion, reducing costly rejects and delays.
• Improves inter-team communication: Design, engineering, quality, and manufacturing teams need to assess corrosion risks. Using a standardized rating system increases clarity and speeds decision-making.
• Cost-effective risk reduction: Early detection of corrosion issues during testing prevents costly recalls or warranty claims. It helps optimize protective coatings and materials before full-scale production.

In short, the GMW15359 standard is important because it standardizes how white corrosion is measured and reported, which improves product quality, ensures supplier accountability, reduces risk, and ultimately protects vehicle durability and customer satisfaction.

As a result, the GMW15359 standard is a critical visual benchmark in General Motors' quality and corrosion assessment toolkit. It helps engineers and suppliers consistently rate white corrosion on zinc-plated parts, ensuring clear communication, compatibility, and long component life.

Our organization, which has been supporting businesses across all sectors for years through a wide range of testing, measurement, analysis, and evaluation activities, has a strong team of employees who closely follow global developments in science and technology and are constantly improving themselves. In this context, we also provide testing services in accordance with the "GMW15359 White Corrosion Rating Scale Guide" standard.