Salt Spray Failure After a Plating Supplier Switch

Background

A bag brand sourced #8 metal zippers with a nickel-plated slider for a commuter backpack line. The zippers were assembled by Donglong Zipper, which bought plated sliders from a subcontractor. When the subcontractor moved production to a new facility, it also changed its plating line chemistry to reduce cost and shorten cycle time.

The brand's specification called for neutral salt spray resistance to ASTM B117 for 48 hours with no base metal corrosion. The previous supplier had consistently met this requirement. The new line was qualified on appearance and thickness but not on corrosion performance, and production resumed without a re-qualification run.

What Went Wrong

Three months after the switch, the brand's own quality lab ran a routine salt spray check on a production sample and found red rust on the slider body at 24 hours. A broader test across retained samples showed failures on most lots produced after the change, while older lots still passed. The failure was consistent enough that it could not be dismissed as a sampling anomaly.

Retail customers had not yet reported corrosion, but the brand had shipped about 40,000 bags with the affected sliders. A field failure would be visible and would likely generate returns, so the brand treated the finding as a potential recall risk and asked for immediate containment.

• Failure: red rust at 24 hours instead of 48.
• Affected volume: about 40,000 bags shipped.
• Older lots: still passing.
• Status: containment required before retail feedback.

Root Cause

The new plating line ran a shorter cycle, which reduced the nickel thickness from the specified range to roughly half of it. Thinner plating gives a shorter path to base metal corrosion, especially at edges and thread roots where coverage is naturally lower. The line also skipped a passivation step that had been part of the old process, which removed residual salts and stabilized the surface.

The qualification process was the deeper cause. The supplier tested appearance, adhesion and thickness but did not run a salt spray test as part of the line change. The brand's specification required 48 hours, but nobody verified that requirement after the process change. The subcontractor treated the move as an internal efficiency project rather than as a change requiring customer approval.

How It Was Resolved

The subcontractor restored the full plating cycle and the passivation step, then submitted samples for a 48-hour salt spray test and a 96-hour extended test. Both passed. Donglong Zipper added a salt spray requirement to its incoming inspection for plated components, with a sample from every lot and a retained record.

For the shipped bags, the brand ran a risk assessment. Because the failure appeared at 24 hours in a laboratory and the bags were used in urban conditions rather than coastal environments, the immediate risk was judged moderate. The brand issued a care advisory to retail partners, extended the warranty on affected units, and offered replacement sliders through its service network. No recall was required.

The brand also added a change-notification clause to its supplier agreements: any change in plating chemistry, line or subcontractor requires written approval and a re-qualification test. The clause now applies to all metal components, not only zippers.

The risk assessment weighed laboratory failure against real use. The bags were sold mainly in urban markets, and the sliders were protected by a flap and a fabric pull. The 24-hour failure was a genuine specification breach, but the probability of visible corrosion within the warranty period was judged moderate rather than high, which is why the brand chose a warranty extension over a recall.

The supplier's re-qualification checklist now has four gates: plating thickness across five points on the part, adhesion, passivation confirmation and a 48-hour salt spray test. Production cannot resume until all four are signed off, and the signed checklist is sent to Donglong Zipper and the brand. The checklist is deliberately short so that it is followed under schedule pressure.

The change-notification clause changed the supplier's internal process. Any move of a plating line, a chemistry change or a subcontractor switch now requires a written request, a sample submission and a re-qualification test before production continues. The clause costs a few days per change, but it prevents a repeat of a three-month silent drift.

The component supplier now keeps a plating lot record that links each production batch to its chemistry, thickness measurements and salt spray result. The record is available to Donglong Zipper on request and to the brand during an audit. That traceability turned a once-opaque process into something a buyer can verify, and it made the supplier more careful about changes, because every change now leaves a visible gap in the record.

Lessons Learned

• Re-qualify corrosion performance after any plating line or chemistry change.
• Specify thickness range and passivation, not only appearance.
• Run salt spray on incoming lots, not only on the first article.
• Require written approval for subcontractor changes.
• Assess field risk by use environment before deciding on a recall.

Corrosion failures are predictable when the process is controlled and invisible when it is not. The new line looked identical in a visual inspection and failed in a chamber. Donglong Zipper now treats a plating change as a design change, and every affected lot carries a salt spray record that the buyer can request.