TR-391 is a two-component blue trivalent chromium passivation made for zinc-nickel alloy coatings. It converts the surface of the plated layer into a dense blue film, and that film carries the corrosion protection on the finished part. No hexavalent chromium is used anywhere in the bath, so nothing in the drag-out or the rinse water has to be reduced back to trivalent chromium before waste treatment.
The chemistry is split across two liquids on purpose. TR-391A builds the film, TR-391B drives the reaction rate and the final shade, and a technician can correct one without unbalancing the other. That is the difference between a passivate you tune and a passivate you replace.
Rack and barrel work run in the same solution. Jigged parts go through the longer rack cycle, bulk work in barrels through the shorter one, and both come out with the same blue finish. Lines that already run zinc nickel plating as a corrosion layer under a decorative top coat use this step to add colour and the last increment of protection in a single pass.
The bath is built to last. It is not consumed by the parts the way a plating electrolyte is, and it absorbs normal drag-out and top-up without falling out of specification, which is what keeps a passivation tank stable across a full shift. When the film starts to come out patchy or pale, the cause is almost always pH or concentration rather than a bath that has worn out.
| Parameter | Setting |
|---|---|
| Product | TR-391 blue trivalent chromium passivation |
| Coating it is applied to | Zinc-nickel alloy |
| Film colour | Blue |
| Components | Two liquids, TR-391A and TR-391B |
| TR-391A | 25 ml/L (operating range 20-30 ml/L) |
| TR-391B | 50 ml/L (operating range 40-60 ml/L) |
| pH value | 4.5 (operating range 4.0-4.8) |
| Passivation time, rack plating | 45 s (operating range 35-55 s) |
| Passivation time, barrel plating | 30 s (operating range 25-35 s) |
| Passivation temperature | 25 °C (operating range 25-40 °C) |
| Line type | Rack and barrel plating |
| Country of origin | China |
Hexavalent chromium is the reason many plating shops are revisiting their passivation line. It is toxic, it is regulated, and the waste treatment it demands — reducing Cr(VI) to Cr(III) before precipitation — is an extra stage with its own chemicals, dosing control and failure modes. This passivate takes that stage out of the flow. What leaves the tank is trivalent chromium, which the existing hydroxide precipitation handles.
For a line converting from a chromate process, the practical changes are smaller than they look. The tank, the rinse stages and the drying oven stay where they are. What changes is the chemistry in the tank, the waste-treatment setpoint and the personal protective equipment around it. It is part of a broader move in metal electroplating away from hexavalent chemistry, and the regulatory pressure behind that move is not going to reverse.
Cost deserves a straight answer, and the straight answer is that it depends on how the line is run. Two components mean two things to analyse and two things to top up, but they also mean the film can be corrected without dumping and remaking the bath. A solution that holds its specification for a long time costs less per part than one replaced every few weeks. Compare consumption per square metre of work, not price per litre.
Documentation matters as much as chemistry when a process changes. Ask your sales contact for the SDS and the compliance documents that apply in your market before the trial rather than after, so the paperwork is ready when the first production batch runs.
What this product will not do is rescue the substrate. A zinc-nickel plating layer that is too thin, or a bath co-depositing the wrong alloy ratio, will still fail a corrosion test after passivation. If the zinc-nickel plating step is also under review, say so at enquiry stage and the two can be looked at together.
Automotive components. Fasteners, brake line fittings, seat frames, brackets and under-hood hardware are the classic home for a zinc-nickel layer with a blue passivate. The combination is chosen where a plain zinc coating will not hold up to road salt and the part still has to look finished. Because the film is thin, thread engagement and torque values do not shift.
Electronics manufacturing. Chassis parts, connector shells, shielding cans and mounting hardware use the same route for protection, and the blue finish reads as deliberate rather than raw. Thin conversion films matter here for a different reason: anything that builds up will not fit a connector or a press-fit hole.
Aerospace components. Ground support equipment, airframe brackets and shop hardware are increasingly specified with trivalent passivates where hexavalent chromium is restricted. The driving question in this sector is documentation and repeatability rather than appearance, so the batch record carries as much weight as the chemistry.
Industrial equipment. Hydraulic fittings, gears, shafts, pump housings and machine guards go through zinc nickel plating and passivation when they are exposed to coolant, washdown or weather. Repair and re-plating work is common here, and a passivate that can be run in small batches without remaking the tank suits that pattern.
Construction materials. Structural fasteners, anchors, window and facade fittings, scaffold couplers and fencing components are usually the largest volume and the least forgiving of a finish that fails outdoors. A blue passivate over a zinc-nickel layer is a practical specification where the part stays visible and the finish has to last.
Across all five industries, the shops that get reliable results treat passivation as part of the plating process rather than as a final dip. Metal plating services that skip that habit usually end up chasing the problem in the seal tank.
Fengfan has been formulating electroplating additives since 1984. Wuhan Fengfan International Trade Co., Ltd is the wholly owned trading subsidiary of Wuhan Fengfan Electrochemical Technology Co., Ltd, and the two together cover research, production and export — which is why a question about a passivation bath is answered by people close to the laboratory rather than routed through a distributor.
The Fengfan Tech Park sits in the Dongxihu District of Wuhan on a site of roughly 67 acres, with about 7,000 m² of floor space and an annual surface-treatment additive capacity of 3,000-5,000 tonnes.
Production runs to written standards: a 6S site-management system, a clean production verification approval and a safety production standardisation programme, with a classified logistics system handling finished goods. When a customer asks why two batches behave the same way, that paperwork is part of the answer.
Development work happens in a dedicated electroplating pilot laboratory, where green process chemistry is trialled on real parts before it reaches a customer's line. That laboratory is also where most customer-specific questions get settled — send a sample and a description of your bath, and the trial happens on a bench before anyone touches a production tank.
For industrial metal plating and industrial electroplating buyers who are used to dealing with traders, the useful part is not the size of the site. It is that the technical answer comes from the organisation that makes the product.
Aerospace parts have to keep their structural integrity in extreme conditions, which is why anodizing aluminium is used to strengthen aluminium components and improve their corrosion resistance at altitude and across wide temperature swings. Alodine coating is applied where paint and other finishes need to bond reliably, and electroplating puts protective layers on the metal parts that complete the assembly.
In construction, anodized aluminium is used on facades and window frames where corrosion resistance and a clean finish both matter, giving a durable surface that suits modern architectural designs. Electroplating strengthens metal fixtures and fittings, and zinc nickel plating adds protection to steel structures against rust so the materials last longer once installed.
Plating does not have to trade environmental responsibility against cost. Our electroless nickel plating services are an environmentally friendlier alternative to traditional plating methods, using less energy and producing less waste, and the savings from a leaner process work in your favour on the final cost. That makes the route attractive for businesses that need efficient metal plating services and want the finish to sit comfortably with their own green initiatives. Tell us about the parts you need plated and we will set out the option that balances performance and cost for your case.
Choosing the right match is where most sourcing decisions stall. As a trading supplier, we keep one point of contact from first enquiry to after-sales: tell us what you are making and the conditions you work under, and we shortlist the closest matches from our supplier network, compare specifications and arrange samples before you commit to a bulk order. If something does not behave as expected, we work with the supplying partner on the cause and a practical fix, and stay with the case until it is settled. Email, WeChat or WhatsApp enquiries get a reply within 24 hours.
It is built for zinc-nickel alloy layers, which is the surface the blue film is designed to react with — it is not a drop-in for a passivate running over plain zinc or zinc-iron. If you are converting from a chromate process, send us the layer thickness and the nickel content you run and we will confirm whether the process matches before you place an order. A coupon passivated in your own bath tells you more in one shift than any data sheet.
Almost always one of three things, in this order: the antioxidant film was not fully removed before immersion, the pH has drifted outside its working band, or the parts were left wet with passivate instead of being rinsed promptly. Check the acid dip and the rinse between the two tanks before you touch the passivate concentration — most patchiness starts upstream of the passivation step in the plating process.
Yes. Both are covered by the same solution and the same concentration; only the immersion time changes, with jigged parts held longer than bulk work in a barrel. If a line runs both on the same shift, keep a separate timer for each so the shorter cycle is not accidentally applied to a rack. Load the barrel so parts do not nest, and make sure jigs do not create a shielded pocket.
On a properly formed zinc-nickel layer, a trivalent blue passivate with a seal stands up to the same test regimes, without the Cr(VI) handling and waste-treatment burden. What it will not do is compensate for a layer that is too thin or a bath co-depositing the wrong alloy ratio. If your current process passes only marginally, review the zinc-nickel plating step at the same time rather than looking for a stronger passivate.
Not always. The blue film works on its own, and many decorative parts are simply passivated, rinsed and dried. Sealing becomes the practical choice when the part carries a salt-spray requirement or will be exposed outdoors, because a chrome-free sealant closes the porosity of the film and adds a barrier of its own. It is routine on industrial metal plating work for automotive and construction hardware.
Same family of chemistry, different film. A black trivalent passivate gives a dark, low-reflectance finish and is usually chosen for appearance; the blue film here gives a brighter, cooler colour and is the more common specification where the part is functional rather than decorative. In metal electroplating the choice is normally written into the drawing, so match what the drawing calls for rather than what looks better on the bench.
Longer than most people expect, because the passivate is not consumed by the parts the way a plating electrolyte is — losses are mostly drag-out and top-up. In practice a well-run tank is maintained rather than replaced: keep the two components in balance, hold the pH in band, control what the rinse water carries in, and the solution stays serviceable. When corrections stop holding, that is the signal to remake it.
Usually yes, because the variables the bath controls — colour, film density and corrosion performance — are set by concentration, time and pH rather than by the customer. What changes between customers is the zinc-nickel layer underneath and the corrosion target they are working to. Run a coupon per specification and keep the results with the batch record; that is how one tank serves several jobs without guesswork.
