The Contract Said 8μm of Nickel. The Bathroom Accessories Measured 3μm
Plating and Coating Thickness Measurement Guide - Why Thickness Comes In Below Spec, XRF, Magnetic Induction, Eddy Current and Cross-Section Methods Compared, How XRF Works and Where It Falls Short, Common Supplier Thickness Tricks, Spec Sheet Items, Stage-by-Stage Checks and a Checklist

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A Korean bathroom accessories brand produced 4,000 sets of towel bars and toilet roll holders at a plated hardware factory in Taizhou, Zhejiang. The spec sheet said "brass base, nickel 8μm min + chrome 0.3μm min." The approval sample the factory sent had a good shine, and its XRF test report showed 9.2μm of nickel, so there was no problem. The pre-shipment inspection passed on appearance and dimensions as well.

From the fourth month of sales, though, reviews kept coming in: "it rusted a few months after I hung it in the bathroom," "the surface is lifting and staining." The brand sent leftover warehouse stock to a test lab in Korea for XRF re-measurement, and the nickel came in at 2.8 to 3.6μm, less than half the contracted thickness. The sample the factory had sent was a separately plated "show piece," and the production goods had a shortened plating time to hit the price.

You cannot see plating or coating thickness. Put a thin layer of chrome over 2μm of nickel and it shines exactly like 8μm. The difference only shows up as rust and discoloration after months of exposure to moisture, sweat and cleaning products. This article starts with why thickness comes in below spec, then covers XRF and the other measurement methods and their limits, the tricks suppliers often use, and how to put thickness measurement into the spec sheet and the inspection checklist.

A reply of "we'll plate it well" or a test report on a single sample is not enough. Set the minimum thickness of each plating layer, the measurement method and locations, and the sample quantity per lot in numbers in the spec sheet, and measure production goods you pick yourself. That is how you prevent thin plating.

1. Why Thickness Comes In Below Spec

Plating thickness is roughly proportional to plating time and current. Cutting time saves nickel, chrome or gold, saves electricity and speeds up line turnover, so for a factory under cost pressure, thickness is the easiest thing to cut. In China sourcing, thin plating usually comes from the following.

• Shortened plating time: The most common cause. Whatever the factory gave up in price negotiation, it makes back in plating time.
• Outsourced plating shops: Most hardware factories send plating to a specialist shop nearby, so even the factory you ordered from often does not know the actual thickness.
• Skipped intermediate layers: The copper or nickel underlayer is left out and only a thin finish layer goes on. It looks the same but corrosion resistance drops sharply.
• Uneven current distribution: Electroplating builds up thick on edges and protrusions and thin inside recesses and around holes. Overloading the racks (jigs) widens the variation.
• Poor paint film control: With powder coating or liquid paint, cutting the number of passes or changing gun distance to save paint makes the film thinner.

Thin plating is a different problem from corrosion testing or adhesion testing. We covered salt spray testing in Episode 128 and paint adhesion testing in Episode 130. Both take time to produce results and destroy the sample. Thickness measurement takes seconds and usually leaves the product intact, which makes it the most practical check to run on every lot at inspection.

2. Measurement Methods — What Measures What

The right method depends on the type of plating or coating and the base material. If you only tell the factory "please measure thickness," it will tend to measure the thickest spot with whatever equipment it has. Decide the method for your product first.

XRF for metal plating, magnetic induction or eddy current for paint

XRF is the standard for metal plating such as nickel, chrome, zinc and gold. Non-metallic coatings like powder coat and paint do not read well on XRF, so use a handheld magnetic induction gauge on steel and an eddy current gauge on aluminum. Combination gauges that do both are common.

Thickness is stated in micrometers (μm)

Plating thickness is handled in units of 1μm (0.001mm). Decorative nickel-chrome usually runs 8 to 20μm of nickel and 0.2 to 0.5μm of chrome. Zinc plating on steel parts is often 5 to 8μm for indoor use and 12μm or more for outdoor use. Gold plating ranges from a 0.05μm "flash" to hard gold of 0.76μm or more, with a price gap of more than ten times. Powder coating is typically 60 to 80μm and aluminum anodizing 10 to 25μm. Because "gold plated" can mean such different things, never sign a contract without numbers.

3. How XRF Works and Where It Falls Short

An XRF gauge fires X-rays at the sample, reads the intensity of the fluorescent X-rays given off by the plating metal and calculates thickness. One reading takes about 10 to 60 seconds, and it can measure several layers at once, such as chrome over nickel, so it is the most widely used tool in plating shops and test labs. Having the equipment does not mean you can trust the number as-is, though. Several conditions affect the result.

Calibration and location matter most. If the factory's XRF is calibrated for a steel base and it measures a brass product, the reading can be off by several μm. Left to the factory, location usually means the flat, thick front face only. When you receive an XRF report, do not just look at the numbers; first check that it states which calibration was used, where it measured and how many times.

4. Common Supplier Thickness Tricks

Because thickness cannot be checked by eye, it is easy to fake. These are the patterns we have actually run into in consultations.

The most common of these is the show piece. The factory knows the inspection date in advance, so it may put thick pieces at the top of a carton before the inspector arrives or prepare separate measurement samples. That is why the inspector must always pick the samples personally, drawing evenly from the top, middle and bottom of several cartons. For the broader problem of samples differing from production, see Episode 34.

5. Defects Caused by Thin Plating

Thin plating hardly shows at shipment. It appears after consumers have used the product for weeks or months.

• Rust and corrosion: Thin plating lets moisture reach the base metal through tiny pinholes. Steel parts get red rust; brass and zinc die-castings get white powder or blister-like corrosion.
• Discoloration: Thin gold or silver plating lets the underlayer show through or quickly turns yellow or black. This is the number one return reason for accessories and jewelry.
• Base metal showing through wear: On frequently touched parts such as handles, buttons and zipper pulls, thin plating wears off quickly and exposes the base color.
• Loss of function: Thin gold or tin plating on electronic connectors raises contact resistance and worsens solderability.
• Regulatory issues: On products that touch skin, a thin finish layer over a nickel underlayer can let nickel leach out and exceed nickel release limits.

Because the defects appear after sale, the costs are larger too. Returns and exchanges, review damage and leftover stock all come at once, and without thickness records from shipment you are at a disadvantage when arguing liability with the factory. For setting defect liability in advance, see Episode 103.

6. Spec Sheet Items

A single line saying "chrome plated" lets every factory pick its own thickness and layer structure. Put the items below in a table and attach it to your spec sheet or quality agreement. For writing spec sheets, see Episode 59.

It is worth getting the plating shop's name too. When your factory switches plating subcontractors, thickness and quality shift together, and without the name you will not even know a change happened. For managing subcontractor changes, see Episode 115.

7. Putting Thickness Measurement into Your QC Checklist

Sample Stage

When you receive the approval sample, have some of it measured by XRF at a third-party lab in Korea or locally, separately from the factory's report. That value becomes the production benchmark. If the cost is reasonable, a single cross-section test also confirms the layer structure and gives you a reference sample to compare against the factory's XRF readings later.

Mass Production Stage

For the first production lot, get the measurement record right after plating, before assembly. Require re-measurement whenever the plating shop changes or the plating bath is replaced. If the values on the record are nearly identical every time down to the decimal point, suspect that they were copied rather than measured. Also confirm at the pre-production meeting that measurement locations and sample quantity went into the work instructions. For running that meeting, see Episode 123.

Pre-Shipment Inspection Stage

The inspector brings a standard of known thickness and first checks that the factory's XRF reads it correctly. Some inspection companies bring a handheld XRF. Next, the inspector measures samples picked personally from production cartons at the locations on the drawing. If even one value falls short, increase the sample size and measure again; if it still falls short, hold the lot. For pre-shipment inspection, see Episode 100.

8. Sample Request Wording for the Factory

Putting the wording below into your purchase order or quality agreement takes thickness and measurement out of the factory's discretion, and makes it easier to assign responsibility if thin plating defects occur.

"产品基材为黄铜，电镀层结构为铜/镍/铬。主要表面任一测量点的镍层厚度不得低于8μm，铬层厚度不得低于0.3μm（最小局部厚度）。镀层厚度采用X射线荧光测厚仪（ISO 3497）测量，测量前须使用与产品相同基材和镀层组合的标准片校准。每批抽取至少5件，每件按图纸标注的正面、边角、凹位3个位置测量，并提交注明批号、样品照片、测量位置和各层数值的测量记录。测量样品由我方验货员从大货中随机抽取。如有一个测量值不合格，须加抽至20件复测；复测仍不合格的批次不得出货。如发生争议，以金相切片法（ISO 1463）结果为准。更换电镀厂或镀液须事先书面通知并重新测量。"
(The base material is brass, and the plating structure is copper/nickel/chrome. At any measurement point on significant surfaces, nickel must be at least 8μm and chrome at least 0.3μm (minimum local thickness). Plating thickness is measured with an XRF thickness gauge (ISO 3497), calibrated before measurement with standards of the same base and plating combination as the product. For each lot, take at least 5 pieces and measure each at the 3 locations marked on the drawing (front, edge, recess), and submit a measurement record showing the lot number, sample photos, measurement locations and the value of each layer. Measurement samples are picked at random from production goods by our inspector. If any value fails, expand to 20 pieces and re-measure; a lot that still fails may not be shipped. In a dispute, the cross-section method (ISO 1463) result prevails. Any change of plating shop or plating bath requires prior written notice and re-measurement.)

9. Three Real Cases

These are anonymized versions of patterns we often see in consultations. Amounts and figures are examples.

Case 1 — Bathroom Accessories

This is the case from the start of the article. The brand sent the re-measurement results to the factory and held it responsible, but the contract did not specify measurement locations or sample quantity, so the factory held out, saying "the spots we measured were over 8μm." The brand eventually got an additional cross-section report, negotiated, and recovered part of the payment, but returns, exchanges and review damage still left a loss of about KRW 25 million. From the next order, it put minimum local thickness and three measurement locations into the spec sheet and had the inspector pick and measure samples personally at pre-shipment inspection.

Case 2 — Gold-Plated Accessories

A company importing "18K gold plated" earrings from an accessories factory in Huadu, Guangzhou received a string of returns saying "the color changed within a month." Lab XRF showed only 0.03μm of gold, essentially flash plating, with the nickel layer showing through. The factory replied, "You only said gold plated; you never set a thickness." The company then specified gold 0.5μm min and a nickel-free underlayer in the spec sheet and required an XRF record for every lot. The unit price rose by about KRW 400, but discoloration returns almost disappeared.

Case 3 — Zinc-Plated Shelf Brackets

A company importing zinc-plated steel shelf brackets from a factory in Hebei received rust complaints from customers who had installed them on balconies. The approval sample had 12μm of zinc, but production goods measured 9μm on the front face and 4μm on the inside of the bend. The racks were packed too tightly, so the plating did not build up evenly on the inner surfaces. The variation came under control only after the inside of the bend was added as a measurement location and the work conditions were changed to hang fewer parts per rack. For managing changes to work conditions, see Episode 119.

10. Common Mistakes

• Writing only the type, such as "chrome plated" or "gold plated," without setting thickness in numbers
• Specifying average thickness so thin areas slipped through
• Trusting a single test report on the approval sample and never measuring production goods
• Measuring samples the factory chose
• Not checking the calibration of the factory's XRF
• Using the wrong method, such as measuring nickel plating with a magnetic induction gauge or paint with XRF

11. Plating and Coating Thickness Checklist

Specification Stage

• Base material and plating layer structure are written down
• Minimum local thickness per layer is set in numbers
• Measurement method, locations and sample quantity are in the spec sheet
• The dispute resolution method (cross-section) is set

Sample and Mass Production Stage

• The approval sample was measured separately by a third-party lab
• The first production lot's record was received with lot number and sample photo
• The plating shop's name and change notification terms are confirmed

Pre-Shipment Inspection Stage

• The factory gauge's accuracy was checked with a standard
• The inspector picked samples personally from production goods
• All drawing locations (including recesses) were measured

Wrapping Up - Check the Numbers, Not the Shine

Every plated or coated part shines the same at shipment. The thickness difference shows up months later as rust and discoloration in the consumer's bathroom or on their wrist, and then returns, review damage and liability disputes all arrive together. Writing minimum thickness per layer and measurement locations into the spec sheet, and measuring production goods you pick yourself with XRF, does not cost much.

There is something you can do right now. Ask the factory currently producing for you to send the plating thickness record for its most recent lot. If there is no record, or it shows only numbers with no lot number or measurement locations, build a plating thickness requirements sheet and send it before your next order.

StartMade supports you on the ground with spec sheets covering layer thickness and measurement conditions, third-party measurement of approval samples, review of measurement records during production, and random-sample measurement before shipment. If you have already received rust or discoloration claims, we start with re-measuring stock and finding the cause. If you are preparing a new product, we start with the thickness requirements sheet.