Does White Staining on Zinc-Coated Steel Mean a Defect? White and Red Rust Inspection Criteria

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When white stains appear on zinc-coated parts during incoming inspection, first distinguish corrosion of the coating from corrosion of the steel substrate. White rust, a zinc corrosion product, and red rust on the steel substrate are different assessment items. Appearance requirements at delivery must also be considered separately before deciding whether to accept the parts and what action to take.

White stains can also be residues or surface contamination. Color alone does not establish that a stain is white rust.

Inspection criteria need to be more specific than “rust is visible.” Defining corrosion type, location, affected area and required exposure time in the purchase specification gives suppliers and buyers a common basis for interpreting results.

How do white rust and red rust differ?

White rust refers to whitish corrosion products that form on the zinc surface. It is distinct from red rust caused by corrosion of the steel substrate. Record its presence and extent as corrosion of the zinc coating when assessing appearance and corrosion resistance.

Red rust consists of reddish-brown corrosion products from the steel substrate. Check whether it occurs at coating damage, uncoated areas or locations where the protective layer has been consumed, and identify features such as threads, edges and contact areas. On hot-dip galvanized steel, rust transferred from another source and brown staining of the zinc–iron alloy layers can also occur. Inspection records should distinguish these forms of contamination or staining from red rust originating in the substrate. AGA guidance on rust staining and coating damage

Delivery appearance and corrosion resistance are separate requirements. A part with no red rust may still fail the customer’s appearance criteria. Conversely, a uniform appearance does not establish compliance with the required corrosion test.

Assess wet storage stain together with the remaining galvanized coating

When freshly hot-dip galvanized parts are tightly stacked while wet, moisture can remain between contacting surfaces and ventilation can be restricted, allowing white corrosion products to form. This is known as wet storage stain. If the changes are concentrated on overlapping surfaces or inside packaging, review stacking, condensation and drainage conditions alongside the condition of the parts on arrival.

Distinguish light surface staining from thick accumulations of corrosion products. Check their distribution, clean the surface as needed, then measure the remaining coating thickness against the applicable specification. During storage, allow ventilation and drainage between parts and prevent prolonged wet conditions. AGA guidance on storing hot-dip galvanized steel

This guidance addresses the storage condition of hot-dip galvanized steel. Assess electrogalvanized parts using their coating thickness, post-treatment specification, appearance requirements and corrosion-test conditions together. Innozinc should likewise be assessed against the applicable product specification and approved test and inspection conditions.

“480 hours of salt spray” needs an assessment criterion

Salt spray test (SST) hours describe the duration of exposure. “480 hours” alone does not indicate whether white rust or red rust was assessed, or which areas were inspected.

ISO 9227 specifies salt spray test methods, but it does not establish the required exposure period or interpretation of results for a particular product. Read the following items together in the test request and report.

Item to check	What to specify for purchasing and inspection
Specimen specification	Substrate, coating type and thickness (μm), post-treatment, and condition of the component or assembly
Test method and duration	Applicable standard and test type, required exposure (h), and intermediate observation times
Corrosion type	Record white rust and red rust separately
Assessment area	Surfaces assessed, affected area (%) or rating, and whether edges, threads and contact areas are included
Acceptance criteria	Limits in the customer specification and appearance reference photographs

If corrosion is reported as a percentage, define whether the denominator is the entire component surface or a designated assessment surface. Whether threads and cut edges are included must also be agreed before testing, rather than decided from the photographs afterward.

“No red rust after 480 hours” and “No white or red rust after 480 hours” are different requirements. Record the test endpoint separately from the time at which corrosion was first observed.

Read corrosion type and distribution in the same 480-hour comparison

The comparison below shows elbow fittings with an 8 μm electrogalvanized coating, an 80 μm hot-dip galvanized coating and an 8 μm Innozinc coating. The test method is ASTM B117. The conditions stated in the source are 35 ± 2°C, NaCl 5 ± 1%, and 480 hours.

Specimen coating	Observation at 480 hours
Electrogalvanizing, 8 μm	White and red rust across the surface
Hot-dip galvanizing, 80 μm	White rust across the surface, with some red rust
Innozinc, 8 μm	No abnormalities observed

The comparison shows not only elapsed time but also the type and distribution of corrosion. At the same 480-hour observation point, widespread white rust can be recorded separately from the presence of red rust.

Innozinc is a composite plating technology combining a zinc layer with an inorganic ceramic film. When reviewing corrosion-resistance requirements, assess coating thickness together with the surface-treatment specification. The photographs and table above show specimens with different coating specifications. Confirm the current supply specification using the Innozinc product information and the evaluation conditions for the intended component.

Link close-up photographs to their location and test history

These close-up photographs show steel grating after 1,152 hours of salt spray exposure. The left specimen has an 80 μm hot-dip galvanized coating; the right has an 8 μm Innozinc coating. At the same exposure time, the surfaces show differences in the accumulation and distribution of corrosion products.

Close-up photographs help identify surface condition. To assess the affected area, link each close-up to the same location in an overall photograph of the component. Include the specimen number and assessment area in the overall view, and record the location and elapsed time for each close-up. Use these records when classifying white and red rust and documenting the affected area.

Also distinguish results for unused finished parts, parts after fastening, and deliberately damaged specimens. A different condition before testing changes the protective performance being assessed. Confirm the condition in which each part was tested, then compare results obtained under equivalent conditions.

Salt spray hours provide corrosion-resistance data under defined conditions. They are not converted into years of field service. ASTM B117’s official guidance also explains the limitations of predicting performance in natural environments from salt spray results alone.

Separate appearance and corrosion criteria in the purchase specification

For appearance, define acceptable color, staining and surface contamination, supported by reference photographs. For corrosion resistance, specify the test method, white- and red-rust requirements, assessment surfaces and acceptance limits. During incoming inspection, checking moisture inside the packaging and the condition of stacked contact surfaces helps document the surface condition at delivery accurately.

Start with these requirements when considering Innozinc. KEMP evaluates samples made from actual components against customer inspection specifications and assembly conditions, then uses the results to establish initial approval and production inspection criteria. The process is explained in From Sample Approval to Production Inspection Criteria (in Korean).

To review a component’s surface condition or corrosion-assessment criteria, send overall and close-up photographs together with the current coating specification and required test criteria. Contact KEMP for sample and specification review

Photographs and test results: Innozinc catalog published on the KEMP website, p. 4 (elbows, 480 hours); Innozinc catalog BV4/May 2022, p. 4 (steel grating, 1,152 hours).