Salt Spray Hours Are Not a Specification: Reading Plating Claims from Chinese Suppliers

I spent eleven days in June of this year in Hebei and Shandong visiting plating shops, and I came back with a notebook full of numbers and a fair amount of humility. Every single shop I visited claimed "salt spray 96 hours" or "500 hours" on their English-language page. Three of them could not produce a test report dated within the last year. One produced a report for a different part number. One produced a beautifully bound report from an accredited lab that had tested a coupon, not the customer's part, on a substrate that was not the customer's substrate.

Salt spray hours are the most quoted and least understood number in hardware sourcing from China. Here is what I check now, in order.

First figure out whether it was a coupon or the part

A test coupon is a flat 100 by 150 mm plate plated along with the batch. A part is the actual bracket with a welded seam, a punched hole, a tight radius and a pressed-in rivet. Both can legally be reported as "480h". The one with a coupon will fail at the edges and the aperture in real life, because plating thickness distribution on a complex part is wildly uneven and salt spray attacks the thin third of the surface, not the average.

Ask for the report on the part. If the lab cannot hold the part on the fixture at the specimen angle the standard requires, ask for a photograph of the fixture with your parts mounted. This question alone kills half the claims I hear.

Read the standard, not the hours

Three numbers explain nearly everything a plating claim is missing.

The first is which standard and which rating scale. ASTM B117 is the neutral salt fog exposure itself. What people call "96 hours" usually means "96 hours, then rate per ASTM D610 for rust area or ASTM D714 for blistering". A 480-hour claim with no rating criteria is a photograph of a wet part. ISO 9227 neutral salt spray is the European version and is broadly comparable, so a supplier citing EN ISO 9227 NaSS at 720h is not using a different language, just a different document.

The second is the substrate. Zinc-nickel on hardened steel, zinc on mild steel and die-cast zinc alloy all behave completely differently. Zinc die castings, very common for padlock bodies and camera housings, will show white corrosion product that many buyers mistake for a plating failure when it is actually the substrate reacting. If a supplier quotes a salt spray hour without naming the substrate, treat the number as marketing.

The third is the acceptance criterion. IEC and most automotive customers define failure as "5 percent of significant surface red rust" for a pass. A report that says "slight rust at 240h" may still pass a 480-hour specification depending on where the rust is. Read the criterion. A supplier who sends you a report with the criterion highlighted understands what they are doing.

The measurement that actually predicts field life

Salt spray hours and plating thickness track each other, but only loosely, and thickness distribution is the variable that matters. On a zinc-plated standard fastener, I want the coating thickness measured in three places: a flat face, a thread flank and an edge radius. If the thread flank is at 3 micrometres while the face is at 12, the 96-hour report is meaningless because the thread is what corrodes.

The practical threshold I use: for interior hardware, 8 micrometres minimum local thickness on zinc with a clear or trivalent passivate. For exterior use, 12 to 15 micrometres, or 8 micrometres of zinc-nickel with a passivate, which gives roughly three to four times the corrosion resistance per micrometre. For marine or road-salt exposure, phosphate plus zinc-nickel plus a seal, or better yet change the design so the part is 316 stainless.

Ask how thickness is measured. The answer should be either X-ray fluorescence on the part at multiple points or the microscopic method on a cross-section. If the answer is "our plating partner guarantees it", ask for the partner's name and their own most recent XRF printout.

Hydrogen embrittlement is the failure that does not show up in salt spray at all

This one deserves its own paragraph because it can kill people. High-strength fasteners, spring steel, and anything hardened above roughly 32 HRC that goes through acid pickling and electroplating will absorb hydrogen. The part passes every salt spray test, passes dimensional inspection, then snaps six weeks later in service with no warning and a brittle, flat fracture face.

The countermeasure is a post-plating bake, typically 4 hours at 200 degrees C within 4 hours of plating for property class 10.9 and above, and the requirement is written into ASTM B850. I have found maybe one in five Chinese plating shops who can show a bake log with a timestamp rather than a sentence in a brochure. If you are buying high-strength fasteners, springs, or anything structural, ask for the bake log and for the 24-hour delayed fracture test the standard specifies.

China RoHS and the paperwork nobody requests

Most plating shops in Zhejiang and Guangdong now produce China RoHS declarations, but the more useful document for a Western buyer is a full material declaration covering hexavalent chromium. Even trivalent passivates, which are the modern default, sometimes sit downstream of a shop that still runs a hex chrome line for other customers, and cross-contamination through shared racks and rinse tanks is real.

Ask specifically: is this piece on a dedicated trivalent line, and can you confirm no hexavalent chromium in the process? Get a recent third-party test, not a declaration letter. A test costs the supplier maybe USD 60 per element set at a domestic lab and takes a week. The ones who do it routinely will already have the file.

A short checklist for your next plating order

Part number, substrate and hardness on the drawing. Plating type and nominal thickness with a specification of where it is measured. Which salt spray standard, what rating criterion, and confirmation the test is on the part. Hydrogen bake requirement and temperature, with a log. Packaging that does not abrade the coating in transit, because a bag of plated parts in a carton that shifts for eight thousand kilometres can arrive with more damage than a year in the field.

And one habit that has saved me real money: keep a sample from each plating batch, labelled, in the office. When a customer complaint arrives two years later, you can put the batch sample next to the failed part and see instantly whether it is a plating problem, a design problem or a manufacturing problem on the assembly line. Salt spray reports tell you about a day in a chamber. A retained sample tells you about your supplier.