Salt Spray Testing of Metallic Coatings on Hardware Components: Standards, Methods, and Evaluation

Salt spray testing of plated hardware components is the most widely used accelerated corrosion test for verifying the protective performance of electroplated and chemically coated finishes on fasteners, hinges, locks, brackets, and other metal hardware. This comprehensive guide explains the testing standards (ISO 9227, ASTM B117, GB/T 10125), sample preparation, test methods, evaluation criteria, and typical duration requirements, helping manufacturers and buyers ensure long-term corrosion resistance and product quality compliance.

Technical Overview

Salt spray testing (also known as salt fog testing) is an accelerated laboratory corrosion test designed to evaluate the corrosion resistance of metallic coatings applied to hardware components. During the test, samples are exposed to a controlled corrosive atmosphere containing a fine mist of neutral or acidified sodium chloride solution inside a sealed chamber maintained at 35 °C or 50 °C, depending on the test method.

For hardware products such as fasteners, hinges, handles, locks, and connectors, the metallic coating—typically zinc, zinc-nickel, nickel-chromium, or chromium plating—serves as a sacrificial or barrier layer protecting the base metal (usually steel, brass, or zinc alloy) from corrosion. The salt spray test accelerates the natural corrosion process so that weeks of laboratory exposure can simulate years of service in humid, chloride-rich environments such as coastal areas, marine installations, automotive underbody applications, and industrial atmospheres.

Although salt spray results do not translate directly into real-world service life, the test remains the industry-standard benchmark for quality control, coating thickness verification, pre-treatment validation, and comparing the relative corrosion resistance of different coating systems. Internationally recognized standards governing the procedure include ISO 9227, ASTM B117, ASTM B368, GB/T 10125, and JIS Z 2371.

Testing Samples

Zinc-Plated Fasteners and Connectors

Bolts, screws, nuts, washers, anchors, and rivets with electroplated zinc or zinc-alloy coatings, frequently supplemented with trivalent chromium passivation. These are the most commonly tested hardware items, with typical requirements ranging from 24 to 200 hours of neutral salt spray before the appearance of white rust, and longer durations before red rust appears.

Decorative Nickel-Chromium Plated Hardware

Door handles, hinges, cabinet pulls, bathroom fittings, and sanitary hardware made of steel, brass, or zinc alloy with multi-layer copper/nickel/chromium plating. These are typically evaluated using the CASS (copper-accelerated acetic acid salt spray) method due to the decorative and functional demands of the finish.

Hot-Dip Galvanized and Zinc-Flake Coated Components

Structural brackets, chain assemblies, clamps, and outdoor construction hardware protected by hot-dip galvanizing or zinc-flake (duplex) coating systems, which generally require extended test durations from 240 hours up to 1000 hours or more.

Anodized Aluminum Hardware

Aluminum profiles, brackets, latches, and CNC-machined parts with anodic oxidation layers, often combined with sealing treatments. These samples are tested according to anodizing-specific acceptance criteria defined in product standards.

Sample Preparation Requirements

• Samples should be representative of production parts, taken directly from the manufacturing line with coatings in their final condition.
• Test surfaces must not be cleaned, abraded, scratched, or handled unnecessarily prior to testing.
• Samples are typically positioned at an angle of 15 to 30 degrees from vertical within the chamber.
• Sample identification must use a method that does not contaminate the test surface or contact other samples.
• Larger components may be tested as complete assemblies, or representative test panels with identical coating processes may be used where permitted by the specification.

Testing Items

The salt spray test program for plated hardware generally includes the following test types and evaluation items:

• Neutral Salt Spray Test (NSS) — 5% sodium chloride solution at pH 6.5–7.2, chamber temperature 35 °C ± 2 °C; applicable to zinc plating, galvanizing, anodizing, and most protective coatings.

• Acetic Acid Salt Spray Test (AASS) — salt solution acidified to pH 3.1–3.3, chamber temperature 35 °C ± 2 °C; provides a more aggressive environment for decorative plating systems.

• Copper-Accelerated Acetic Acid Salt Spray Test (CASS) — salt solution with copper(II) chloride addition at pH 3.1–3.3, chamber temperature 50 °C ± 2 °C; used mainly for decorative copper/nickel/chromium coatings on hardware and sanitary fittings.

• White Rust Resistance — time until the appearance of zinc corrosion products (white deposits) on zinc-based coatings.

• Red Rust Resistance — time until corrosion of the base metal (iron rust) appears, indicating coating failure.

• Optional complementary items: coating thickness measurement, adhesion testing, and appearance assessment before and after exposure.

Testing Methods

Standard References

• ISO 9227 — Corrosion tests in artificial atmospheres: salt spray tests.
• ASTM B117 — Standard Practice for Operating Salt Spray (Fog) Apparatus.
• ASTM B368 — Standard Test Method for Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing (CASS Test).
• GB/T 10125 — Chinese national standard for corrosion tests in artificial atmospheres, technically aligned with ISO 9227.
• JIS Z 2371 — Japanese standard for salt spray testing methods.

Test Procedure

The salt solution is prepared by dissolving sodium chloride in deionized water at a concentration of 50 ± 5 g/L, with strict limits on impurities. The solution pH is adjusted according to the selected method (neutral or acidified). The chamber is pre-conditioned to the specified temperature, and samples are placed on non-corrosive racks so that the mist settles uniformly on all test surfaces without interference between specimens.

Fog collection rate must be maintained at 1.0–2.0 mL per hour per 80 cm² of collection area, and the collected solution must match the specified concentration and pH. Tests run continuously for the specified duration—common checkpoints include 24, 48, 72, 96, 120, 240, 480, 500, 720, and 1000 hours—during which samples may be inspected at intervals without disturbing the test surface.

Evaluation of Results

After exposure, samples are gently rinsed with clean water and dried before examination. Assessment is performed according to ISO 10289 or ASTM B537, which define rating numbers based on the percentage of the surface affected by corrosion:

• Rating 10 — no corrosion defects; perfect surface.
• Ratings 9 to 6 — increasing percentages of corroded area, from approximately 0.1% up to 10%.
• Ratings 5 and below — heavy corrosion covering more than 10% of the surface; generally unacceptable for most hardware specifications.

For zinc-plated hardware, buyers commonly specify both a white-rust requirement (for example, no white rust after 72 hours) and a red-rust requirement (for example, no red rust after 120–200 hours), depending on coating thickness and passivation type. Trivalent chromium passivations with sealers routinely achieve 120–300 hours to white rust, while zinc-nickel alloys can exceed 720 hours to red rust.

Typical Duration Requirements by Application

• Indoor furniture and general hardware: 24–48 hours NSS, no red rust.
• Outdoor hardware and general fasteners: 72–120 hours NSS, no red rust.
• Automotive fasteners and chassis hardware: 240–720 hours NSS or cyclic testing, no red rust.
• Marine and coastal applications: 500–1000 hours or more, verified with cyclic corrosion tests.

Applications

Salt spray testing of hardware coatings is applied across many industries and product categories:

• Fastener industry — quality control of zinc, zinc-nickel, and zinc-flake coatings on bolts, screws, nuts, and washers supplied to automotive, construction, and machinery customers.

• Architectural and door/window hardware — validation of nickel-chromium, PVD, and satin finishes on locks, handles, hinges, and closing systems for corrosion resistance in humid service environments.

• Furniture and kitchen/bathroom fittings — verification of decorative plating against humidity, condensation, and cleaning-agent exposure using CASS and combined climate tests.

• Automotive supply chain — incoming and outgoing inspection of electroplated hardware according to OEM specifications, often supplemented by cyclic corrosion tests.

• Electronics and electrical hardware — assessment of plated terminals, shields, and connector housings against galvanic and atmospheric corrosion.

• Marine, coastal, and infrastructure hardware — qualification of heavy-duty protective systems such as hot-dip galvanizing and duplex coatings for extended service life.

FAQ

How long does a salt spray test for plated hardware take?

The duration depends on the coating system and the application requirement. Common durations range from 24 hours for basic indoor hardware to 1000 hours or more for marine-grade and automotive coatings. The test runs continuously, so a 240-hour test, for example, takes ten full days in the chamber.

Does a longer salt spray time guarantee a longer service life?

Not necessarily. Salt spray testing provides a comparative, accelerated assessment of corrosion resistance under standardized chloride exposure, but it does not directly predict service life in real environments, where factors such as UV radiation, temperature cycling, wet-dry alternation, and mechanical wear also play major roles. For more realistic evaluations, cyclic corrosion tests that alternate salt spray, humidity, and drying phases are increasingly specified.

What is the difference between white rust and red rust?

White rust consists of zinc corrosion products that form on zinc-based coatings and indicates the passivation layer is degrading; the coating can still protect the base metal. Red rust is iron oxide from the underlying steel, which means the coating has failed and the base metal is corroding. Specifications therefore usually set separate time requirements for each.

Which test method should be chosen: NSS, AASS, or CASS?

NSS is the default choice for protective coatings such as zinc plating, galvanizing, and anodizing. AASS is a more aggressive variant used for certain decorative systems. CASS, with copper acceleration and a higher temperature, is specifically intended for decorative copper/nickel/chromium plating on hardware and sanitary fittings, where it reveals porosity and thickness deficiencies quickly.

How should samples be submitted for testing?

Samples should be taken from normal production, kept in their as-delivered condition without cleaning or touching the test surfaces, and accompanied by information on the substrate material, coating type, nominal coating thickness, passivation, and the required test standard, method, and duration. Independent accredited third-party laboratories can then execute the test and issue a formal report with photographic documentation and rating results.