A batch shows white rust after 96-Hour Salt Spray. Can it ship?

Answer: Judge by area. Under 5% in a 1 dm² frame, parts pass after washing and passivation if torque retests at 0.55 N·m; above 30%, replace the batch.

Why not clean hardware with hydrochloric acid?

Answer: It dissolves zinc over 10 times faster than citric acid: at 5% it opens an 8 μm coating in 3 min, and the bright finish is exposed steel.

Is 480 hours needed on top of 96?

Answer: Inland storage settles at 96 h; offshore platforms and deck fittings run to 480 h or 720 h, where the criterion shifts from white rust area to red rust presence.

Can passivated parts be painted at once?

Answer: Yes, after checking the film type. Trivalent chromium bonds best to powder coatings within 24 h; past 72 h the film ages and adhesion falls 20%.

Torque wrench or torque screwdriver for the retest?

Answer: Below M4 use a 0.1–1 N·m screwdriver rated ±3%, above M6 a wrench, one tool per batch, since two tools disagree by 0.05 N·m.

How is coating thickness measured without cutting a part?

Answer: A magnetic gauge on flat surfaces, 3 readings averaged. Skip edges and threads, where readings run low and good parts get rejected. ---

Protective Case Hardware After 96-Hour Salt Spray

Quick answer: After 96-Hour Salt Spray, hardware is graded before cleaning: white rust can be washed, red rust polished, exposed substrate replaced. Wash in 5% citric acid at 40 °C for 20 min, passivate for 3 min, retest torque at 0.6 N·m, and replace hinge pins and springs whatever the grade.

Grade the hardware before it is cleaned
Citric acid: concentration, temperature, time
Passivation and relubrication close the coating gaps
Torque retest and the parts that must be replaced
Three stainless grades and zinc-nickel compared
FAQ

1. Grade the hardware before it is cleaned

Chamber parameters are fixed: 5% sodium chloride, 35 °C, pH 6.5–7.2, deposition 1–2 mL/(80 cm²·h). After 96-Hour Salt Spray the first job is grading, not washing; rust wipes off in one pass and takes the evidence with it.

Grading reads colour, area and substrate exposure. White rust is powdery zinc oxide that lifts under a fingernail; red rust is brown iron corrosion that does not lift; exposed substrate means the coating has perforated and grey steel shows under a loupe.

Grade	Appearance	Area within 1 dm²	Action
1, white rust	Pale powder, lifts easily	Under 5%	Wash, reinspect
2, red rust	Brown pits, does not lift	Under 1%, spot under 1 mm	Polish, passivate, non-load positions
3, substrate	Coating perforated, grey steel	Any	Replace

The 1 dm² frame ends the argument: one spot is 0.2% of a lid and 8% of a latch seat.

2. Citric acid: concentration, temperature, time

Hydrochloric acid is the wrong tool: at 5% it opens an 8 μm coating in 3 min, and the bright surface left behind is bare steel. Citric acid at 5% chelates instead, sits near pH 2.2 and attacks the oxide, not the coating.

96-Hour Salt Spray samples in an olive green case open on a steel bench, wrenches and sockets set in black foam, shelving behind

The recipe is 5% citric acid, 40 °C, 20 min, 2 g/L of inhibitor. Below 30 °C the reaction stalls and half the white rust survives; above 50 °C the rate triples and the coating loses more than 2 μm.

Spring parts stay out of the ultrasonic tank: at 40 kHz and 0.5 W/cm², 8 min matches a 20 min dip, but 65Mn leaves lose 6% of load after 10 min. Rinse 60 s in tap water, then 30 s in deionised water below 10 μS/cm.

3. Passivation and relubrication close the coating gaps

Washing removes rust but restores no coating. Trivalent chromium at 25 °C for 3 min, pH 4.0–4.5, builds a 0.2–0.4 μm film worth another 96–120 h; a molybdate system is 20% weaker but free of discharge limits.

Step	Parameters	Time	Check
Citric wash	5%, 40 °C, pH 2.2	20 min	No white rust in frame
Two-stage rinse	Final stage under 10 μS/cm	60 s plus 30 s	No drying mark
Passivation	Trivalent chromium, 25 °C, pH 4.0–4.5	3 min	Film 0.2–0.4 μm
Drying	60 °C	15 min	No runs or stains
Relubrication	0.03 g lithium grease per pin	At assembly	Free movement

Dry within 30 min at 60 °C for 15 min, then relubricate: 0.03 g of lithium grease per hinge pin, 0.01 g on the latch pivot, silicone on the valve O-ring, never mineral oil, which swells a nitrile ring 8% in 3 months. Under 5 μm of coating, white rust returns within 96 h and the part is replaced, not washed twice.

4. Torque retest and the parts that must be replaced

Torque is where damage hides. The M4 insert is specified at 0.6 N·m. Yet after 96-Hour Salt Spray it strips at 0.45–0.5 N·m. Corrosion has taken coating from the thread root, and clearance has grown from 0.05 mm to 0.12 mm. Tighten, release, repeat 3 times. A drop above 10% means a new insert.

96-Hour Salt Spray close-up of a brushed stainless latch over a grey tray corner, tongue on the lid rim, grey dividers inside

Four items go without discussion: hinge pins worn 0.1 mm or with bore clearance past 0.15 mm; springs whose 65Mn free length shifts 3% or whose load falls 15%; locks with key insertion above 15 N or a dragging barrel; valve O-rings with any crack or set. All four fail invisibly, so they are changed on schedule, not on inspection.

Latch bodies and hinge seats survive under 1% red rust: polish, passivate, touch in paint, move to a non-load position. Above that, scrap them.

5. Three stainless grades and zinc-nickel compared

Four finishes separate at 96 h. Electroplated zinc whitens in 24–48 h, 304 pits from 48 h, 316 holds to 200 h, and zinc-nickel runs to 720 h in 5% sodium chloride at 35 °C. Molybdenum and nickel explain the spread: 316 carries 2%–3% Mo against chloride pitting, while 12%–15% Ni zinc-nickel builds a dense product that plugs its own pores.

Finish	After 96 h	Replacement cycle	Cost index
Electroplated zinc on steel	15%–30% white rust	After each run	1.0
304 stainless	3–8 pits per dm²	2 years	1.4
316 stainless	No rust, dulled only	5 years	2.1
Zinc-nickel	No white rust, slight edge tint	4 years	1.8

Inland duty is served by 304; coastal work needs 316 or zinc-nickel, 20% dearer and repaid in saved labour.

Conclusion

The order after 96-Hour Salt Spray is grade, wash, passivate, retest. Three lines cover the criteria: white rust under 5% in a 1 dm² frame is washed and returned to service; red rust under 1% goes only to non-load positions; hinge pins, springs, locks and valve O-rings are replaced outright. Hardware is a small share of case cost, but the torque retest decides whether the case still keeps water out.