Corrosion Testing for Metal Buttons: Salt Spray and Perspiration

A metal button corrosion test answers one question before production rather than after a complaint: will the finish still look like a finish after a season of wear. Plating on a brass or zinc alloy snap is porous at the microscopic level, and the base metal beneath it corrodes in the presence of moisture and salt.

This guide covers the two tests that appear on accessory orders most often, the salt spray test button standard and the perspiration resistance button method. It sets out the solutions, temperatures and durations, the rating systems, the failure modes to look for, and the way to write a requirement so a supplier and a buyer measure the same thing.

1. Why a Metal Button Corrosion Test Matters

Metal corrodes because an electrical circuit forms across a thin film of moisture. The button sits against fabric that holds that film, and the wearer supplies both sweat and movement. Once the film bridges two metals of different potential, the less noble one dissolves.

On a plated part the film does not have to breach the plate to cause visible damage. A pore 1 micron across is enough to start a corrosion cell, and the oxide it produces spreads sideways under the plate. The result is a ring of blistering around a defect far too small to see.

The commercial consequence scales with the application. A shirt snap that dulls over a year is a cosmetic matter, while a bag fitting that pits through and releases its contents is a return. Testing settles which band a part sits in before the order is placed rather than after the container has sailed. Difei Button quotes a tested duration beside each plating option so the choice can be made on a figure rather than on an impression.

2. Salt Spray Test Button Procedure and Standards

The salt spray test button method is defined by two standards that behave similarly in practice. ASTM B117 and ISO 9227 both specify a neutral salt solution of about five percent sodium chloride, a chamber held at 35 degrees Celsius, and a continuous fog for the whole of the test. A salt spray test button run therefore reports four figures that must travel together, which are the solution, the temperature, the collection rate and the duration.

The parts are prepared in a way that matters as much as the chamber setting. Samples are degreased, and they are placed on a rack at between 15 and 30 degrees from vertical so that no part shadows another and no droplet runs from one specimen onto the next. A part laid flat collects solution and fails earlier than the same part hung correctly.

Duration is what the buyer selects, and it should follow from the plating rather than from habit. A thickness of 2 microns and a thickness of 5 microns cannot be judged against the same number of hours, so the requirement is written as a thickness plus a duration rather than as a duration alone.

3. Neutral Salt Spray, Acetic Acid and CASS

Three variants of the salt spray method cover different finishes, and naming the variant prevents a result being read against the wrong scale. The neutral test runs at a pH close to 7 and suits most plated garment parts. The acetic acid version lowers the pH to about 3.1 and accelerates attack on a decorative plate.

The copper accelerated version adds copper chloride to the solution and raises the chamber to 50 degrees Celsius, which makes it the quickest of the three. It is used mainly on decorative chromium finishes, where a normal neutral test would take several hundred hours to produce a readable result.

A duration figure is meaningless without the variant beside it. Forty eight hours of neutral spray is a modest requirement, while forty eight hours of copper accelerated spray is severe, and the two figures appear in quotations without qualification often enough that buyers compare them as if they were the same thing. Difei states the variant beside the hours on every quotation for that reason.

4. Perspiration Resistance Button Testing

A perspiration resistance button test addresses the environment a salt chamber does not reproduce: an acidic film held against the metal at body temperature for hours at a time. The standard artificial sweat solution used on accessories is buffered to either an acidic or an alkaline pH, and the choice of solution follows the garment type.

The procedure is straightforward. A specimen is placed in contact with the solution in a container held at 37 degrees Celsius for about four hours, then removed and inspected. Contact with skin means an acidic solution at a pH near 5.5, while a part that sits against a garment which is itself in contact with skin is often tested in the alkaline solution at a pH near 8. A perspiration resistance button result is reported beside a metal button corrosion test result rather than instead of it, since the two answer different questions about the same part.

The perspiration test and the nickel release calculation are related but not the same. The sweat solution ages the part and reveals discolouration, while the release figure is measured separately and is quoted per unit area per week. A part can pass the visual check and still fail the release limit, so both figures belong in the specification where skin contact is involved.

5. Rating a Metal Button Corrosion Test: How Results Are Scored

Rating is where two laboratories can disagree unless the method is named. The common approaches score the area of corrosion, the size and density of blisters, or the spread of corrosion measured outward from a scribed line, and each produces a different number from the same specimen.

A percentage area scale runs from a clean surface to extensive attack and is read against a reference chart. A scribe test cuts through the plate to the base metal and measures how far the corrosion creeps from the cut in millimetres. A blister scale grades the size of the raised spots on a scale from fine to large.

A purchase order should therefore name the standard, the variant, the duration and the rating scale together. A requirement written as forty eight hours with no further detail leaves the laboratory free to apply any of the five methods above, and a supplier to report the most favourable one. That is the commonest reason two metal button corrosion test reports on the same part cannot be compared.

6. Sample Preparation for a Salt Spray Test Button Run

Preparation decides the result almost as much as the chamber does, and it is the step most often skipped when a part is tested in a hurry. A specimen that has been handled without gloves carries a film of skin oil, and a fingerprint on a plated surface becomes a corrosion site within hours of the test starting.

The sample should be representative of production rather than of the sample room. A part pulled from a golden sample may carry a heavier plate than the average of the run, and a part cut from a sheet may differ from one formed on the production press. Difei Button draws test specimens from the running lot rather than from the approval sample for that reason.

Fixtures matter too. Parts are hung on inert plastic or nylon coated wire, and metal hooks are avoided because a copper or steel hook creates a galvanic pair with the specimen. Where several parts are tested together, they are spaced so that one does not shield another from the fog. A salt spray test button order that names the fixture as well as the duration removes one more variable from the comparison.

7. Common Failure Modes in a Metal Button Corrosion Test

Failures fall into a small number of patterns, and each points to a different correction. Blistering with no visible pit usually means the plate is too thin or the pre-treatment was incomplete. A dark stain that spreads from an edge points to a pore or a scratch acting as the entry for the cell. Where a perspiration resistance button sample fails in the same place as a spray sample, the fault sits with the plate rather than with the environment.

White or grey powder on a zinc alloy part indicates that the alloy itself is corroding, and the correction there is either a heavier plate or a change of base metal. A green stain near a brass part suggests copper is migrating and the lacquer, if there is one, has been breached.

The timing of the failure carries information as well. A part that fails within the first eight hours of a neutral spray has a fundamental problem with the plate, while one that survives forty eight hours and fails at seventy two has a marginal thickness that a small increase will address.

8. Writing the Requirement into a Purchase Order

The requirement belongs in the specification rather than in correspondence, and it reads as a set of linked figures. Name the standard, the variant, the duration, the rating method and the acceptance threshold in one line, and the laboratory has no room to interpret.

A workable clause for a garment snap reads as a neutral salt spray to ISO 9227 for 48 hours with an area of corrosion below 1 percent and no blister above the fine grade, plus a nickel release below 0.5 microgram per square centimetre per week where the part touches skin. Written that way, two suppliers return results that can be compared.

Acceptance thresholds should follow the application. A part worn next to the skin needs a tighter surface requirement than one on an outer flap, while a marine or outdoor application needs a longer duration than either. Difei Button (Guangzhou) Co Ltd reports the plating thickness and the tested duration on each batch certificate, so a buyer can check that the delivered goods match the figure that was quoted.

9. Frequently Asked Questions

9.1 How long should a salt spray test button run?

Duration follows from the plating thickness and the application rather than from a default figure. An inner garment closure commonly runs 24 hours of neutral spray on a plate of 1 to 2 microns, while a bag fitting runs 72 hours on 3 to 5 microns and a marine part runs 96 to 120 hours on a heavier plate. Quoting a duration without the thickness beside it makes the result hard to compare, since the same number of hours means different things at different plate depths. A buyer who specifies 48 hours should expect a heavier plate to be needed than on a part quoted at 24 hours in the same alloy.

9.2 When do I need a perspiration resistance button test as well?

Order the perspiration test wherever the part sits against skin or against a garment that does, because a salt chamber does not reproduce an acidic film held at body temperature. The artificial sweat solution runs at 37 degrees Celsius for about four hours at a pH near 5.5 for skin contact or near 8.0 for a part against the garment. A part that passes the visual check can still fail the nickel release limit, so both figures belong in the specification. Where a style ships to a market with a release limit, the two results are normally requested together on a single report.

9.3 What does a metal button corrosion test result actually tell me?

It tells you how the finish behaves under a standardised attack, which is useful for comparing two suppliers and two plating thicknesses on the same basis. It does not predict service life in weeks or months, because the chamber applies a constant fog that no garment sees. Read the result as a rank rather than as a calendar, and set the acceptance threshold from the application rather than from the highest figure a supplier offers. A result quoted at 48 hours on a 2 micron plate says more about the plating line than about the wearer.

9.4 Why do two laboratories report different results on the same button?

Differences come from the method rather than from the part in most cases. The standard, the variant, the chamber temperature and the rating scale all change the reported figure, and a result quoted without those details cannot be reproduced. Sample condition adds a second variable, since a part handled without gloves or drawn from a golden sample behaves differently from one taken from the running lot. Naming the full method in the request removes most of the disagreement, and asking the laboratory for the chamber settings alongside the result closes the gap before it appears in a dispute.