In case hardware coating, where do zinc and nickel differ?

Answer: In mechanism. Zinc works sacrificially with late red rust, reaching 300 h at 10 μm; nickel works as a barrier with no white rust at 240 h but pitting at any pore.

Does a thicker coating always resist corrosion better?

Answer: Returns flatten past 15 μm. Zinc from 8 μm to 12 μm lifts first red rust from 240 h to 360 h, while 12 μm to 16 μm adds only 40 h.

Do stainless parts still need surface treatment?

Answer: Yes, passivation. Twenty minutes in nitric acid lifts 304 first rust from 240 h to above 500 h; without it, machining residue starts pitting.

How should hydrogen embrittlement be written on a drawing?

Answer: Bake above 1000 MPa parts at 200 ℃ for 4 hours within 1 hour of plating, test five parts to ASTM F1940, and reject the lot on failure with no second bake.

Can one case mix different coatings?

Answer: With the potential gap controlled. Zinc against stainless exceeds 0.25 V and the zinc part corrodes 2.5 times faster; an insulating washer is the simpler answer.

Case Hardware Coating Options

Quick answer: Under ASTM B117 and ISO 9227 neutral salt spray, case hardware coating is judged by two separate hours: first white rust and first red rust.

Three coating routes for case hardware
Reading B117 and 9227 ratings
Coating thickness against current density
Hydrogen embrittlement in high-strength fasteners
Cost and cosmetic consistency
Choosing a coating grade by environment
FAQ

1. Three coating routes for case hardware

ASTM B117 and ISO 9227 run identical conditions: 35 ℃, 5% sodium chloride, pH 6.5 to 7.2, and 1 mL to 2 mL per 80 cm² per hour. Fix the conditions and readings compare across labs; a single "salt spray hours" figure invites argument.

Zinc protects sacrificially, so white rust arrives early and red rust late. Nickel is a barrier, dense and barely sacrificial, pitting at the first pore. Stainless relies on a chromium oxide film above 18% chromium that re-forms after damage.

Route	Typical thickness	First white rust	First red rust	Cost index
Alkaline zinc	8–12 μm	72–120 h	240–360 h	1.0
Acid zinc	5–8 μm	48–72 h	168–240 h	0.9
Electroplated nickel	10–15 μm	above 240 h	above 480 h	2.6
304 stainless	none	not applicable	above 500 h	3.8

A 2 μm difference in zinc thickness moves first red rust by 90 hours, so any case hardware coating report must state thickness beside hours.

2. Reading B117 and 9227 ratings

Rate on ISO 10289 ten-point scales rather than pass or fail. Rp covers substrate corrosion and Rr covers coating white rust, written as a pair such as Rp 9 / Rr 7: 0.25% to 0.5% of the substrate corroded, 1% to 2.5% of the coating bloomed.

Rest plates at 15° to 30° so condensate drains without washing off the fog. Flat plates collect salt and corrode 1.4 times faster, and samples need 30 mm gaps.

Read at 48, 96, 240 and 480 hours instead of only at the end, because first white rust appears between readings. On a third-party report, check the chamber deposition log, the measured thickness and the magnification of the rating photograph.

3. Coating thickness against current density

Thickness is current density multiplied by time. Alkaline zinc at 2 A/dm² deposits 0.33 μm/min, so 30 minutes gives 10 μm; acid zinc at 3 A/dm² reaches 9 μm in 18 minutes but throws poorly, leaving recesses 30% thin.

Case hardware coating samples: four small cases on pale wood shelving, two black and two blue, red latches, metal instruments in moulded inserts

Bath	Current density	Temperature	Deposition rate	Tolerance
Alkaline zinc	1–3 A/dm²	25–30 ℃	0.33 μm/min	±2 μm
Acid zinc	2–5 A/dm²	20–28 ℃	0.50 μm/min	±2.5 μm
Electroplated nickel	2–5 A/dm²	50–60 ℃	0.20 μm/min	±1.5 μm
Electroless nickel	not applicable	85–90 ℃	0.015 mm/h	±1 μm

Corners draw 20% to 40% more current than faces and run 3 μm to 5 μm thicker, so a face that just reaches 10 μm leaves corners at 14 μm. Measure five points; a 4 μm spread means racking or anode area needs work first.

4. Hydrogen embrittlement in high-strength fasteners

Above 1000 MPa tensile strength, plating hydrogen migrates into the lattice and parts snap 24 to 72 hours after assembly with no visible deformation. The failure share is small, the consequence is not, so the specification needs its own clause.

Case hardware coating reference: black waterproof case open on grey studio background, two black latches and silver hinge pins, flocked foam insert

Bake within 1 hour of plating at 200 ℃ for 4 hours, then cool below 80 ℃. Wait 8 hours and efficiency halves; above 12 μm, extend the bake to 6 hours. Test to ASTM F1940 at 75% of tensile strength for 200 hours.

Stainless skips plating, yet cold-headed screws keep residual stress. After three 180° bends in coastal air, 304 pitting potential drops 200 mV, which only 316 or a solution anneal holds back.

5. Cost and cosmetic consistency

An 8 mm × 20 mm latch pin costs 0.8 CNY in alkaline zinc, 2.1 in electroplated nickel, 3.2 in 304 and 4.6 in 316, which is why cheap hardware is almost always zinc plated.

Nickel holds Ra 0.2 μm to 0.4 μm and ΔE within 1.5 across three racks. Zinc passivation ranges from blue-white to iridescent yellow with ΔE up to 4, so two tones in one delivery are common.

Never mix grades on one case. A stainless hinge beside a zinc latch exceeds 0.25 V potential difference, and the zinc part then corrodes 2.5 times faster.

6. Choosing a coating grade by environment

Choose by environment rather than unit price, then fix four acceptance items: five-point thickness, salt spray rating, 1 mm cross-hatch adhesion and ΔE.

Environment	Recommended grade	Reason
Indoor storage, ≤60% RH	Alkaline zinc 10 μm	First red rust at 300 h, five years of appearance
Coastal transit, 85% RH	Electroplated nickel 12 μm	Barrier coating, no white rust at 240 h
Cold chain with condensation	304 stainless	Self-healing film, no coating to flake
Chemical plant, de-icing salt	316 stainless	2%–3% molybdenum, pitting resistance 25

Cold chain work also needs a condensation count. Three moves a day from -18 ℃ to room temperature is 1000 cycles a year: zinc blooms at month 8, nickel holds to month 20, and 316 stays clear at month 36.

Conclusion

Grade by environment first, material second, price last: indoor storage on 10 μm alkaline zinc, coastal and condensing sites on 12 μm nickel, cold chain and chemical exposure on 304 or 316. Then write four acceptance items onto the drawing: five-point thickness, ISO 10289 rating, 1 mm cross-hatch and ΔE 1.5. The price spread looks like 2.6 to 3.8 times, yet on a finished case it is often 6% of unit cost, while one corrosion complaint from a coastal customer costs more than that in replacement and freight. What case hardware coating really guards against is not salt fog but three coatings mixed on one case and accepted at a single price grade.

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.