PN-7 is a chrome-free sealing agent applied at the end of a zinc electroplating line, after passivation and before the drying oven. It goes on as a dilute water-based bath at room temperature, and once the part is dried it leaves a thin, transparent organic film over the passivated surface.
That film is the product. Passivated zinc is a thin, chemically active layer, and in a metal plating process it has to survive handling, packing and transport long after it leaves the tank. This sealant binds that layer and keeps moisture away from it without recoloring the part — the passivation keeps its blue, white or color tone, and keeps it for longer than an unsealed part would.
The bath suits rack and barrel lines, and it is aimed at parts that are seen as much as used: fasteners, brackets, connector shells, fittings and architectural hardware. For buyers sourcing plating chemicals who are moving a chrome-containing post-treatment off the line, this is the step where the chromium comes out without the appearance changing with it.
Because the film is organic and transparent rather than a conversion coating, it does not build a visible layer or shift the color of the passivation underneath. What it does need is properly alkaline make-up water and a controlled concentration — both covered below. Electronics manufacturers also use it over gold plating, where the film keeps conductive surfaces reliable and long-lasting. Formulators that supply surfactants can build it into their own blends, where it improves adhesion and the uniformity of the coating.
Sealing is a short step in the metal plating process, but it is the one that decides what a passivated part looks like when it reaches the customer. A passivation layer is only microns thick and it is porous at the micro scale; once it is dry and exposed, humidity, fingerprints and abrasion all start working on it. This sealant closes that window by wrapping the part in a continuous organic film that limits how much moisture and oxygen reach the passivated surface underneath.
The second mechanism is mechanical. After drying at 60–80 °C the film adds surface hardness and scratch resistance, so parts that rub against each other in a box or a barrel come out looking the way they went in. That matters most on small parts finished in bulk — screws, terminals, small stampings — where sealing is also the metal plating line's last chance to prevent cosmetic damage before packing.
There is a time element too. In any plating process, passivated zinc that is left unsealed drifts in color, particularly the blue and color tones, as the layer oxidizes in storage. Sealed parts hold their tone considerably longer, which is what lets a plater promise a color match on a repeat order placed months after the first one.
None of this replaces good practice upstream. The passivation has to be sound before sealing, the part has to be properly rinsed, and the bath has to be kept at the right concentration. This sealant protects a good passivation layer; it does not repair a poor one.
| Product type | Chrome-free organic sealing agent |
| Film appearance | Transparent, colorless |
| Working concentration | 5–10% sealant in water |
| Make-up water | pH above 7.0 before the sealant is added |
| Bath pH | 8–9 |
| pH adjustment | Diluted ammonia or ethanolamine |
| Operating temperature | Room temperature |
| Immersion time | 5–15 seconds |
| Drying temperature | 60–80 °C |
| Tank material | Stainless steel, plastic, or steel with a plastic liner |
| Barrel plating | Separator required |
| Hexavalent chromium | Not contained |
The economics of a sealing step are easy to misjudge, because the sealant is a small line item next to the zinc and the passivation. What it actually controls is the reject rate. A passivated part that scuffs in the box or discolors in storage has already been through the whole line, and now it has to be stripped and run again — the most expensive outcome in a metal plating shop.
This sealant keeps that cost down in several ways. It runs at room temperature, so there is no heating duty on the seal tank, which is unusual for a post-treatment that has to do real work on the surface. It needs 5–15 seconds of contact, so it sits in the line at whatever speed the line already runs without becoming a bottleneck or forcing a buffer.
The working bath is dilute, and the part leaves with only a thin wet film on it, so drag-out is small and the bath holds its concentration across a long production run. Additions are made on concentration rather than on a fixed schedule, so chemistry is not going into a bath that does not need it.
The last piece is downstream. Parts that are sealed and properly dried resist scratching and keep their color, so they arrive at the customer's incoming inspection in the condition they left your line. Fewer cosmetic rejects at the far end is where the return on a sealing step actually shows. When plating chemicals are compared on landed cost per litre alone, this is the line that usually gets left out of the comparison — and it is the one that decides whether the rest of the spend was worth it.
In automotive manufacturing, the treatment suits vehicle components that have to keep their finish through road salt, moisture and repeated washing, including parts finished on zinc-nickel and electroless nickel lines. The transparent organic film helps those parts stand up to harsh conditions, which means fewer corrosion complaints, lower maintenance costs and better satisfaction further down the supply chain.
PN-7 takes chrome out of the post-treatment step entirely. That makes the process safer for the people running the line and easier on the waste stream leaving the plant, without asking anyone to settle for a lower-quality finish. What stays behind on the plated part is a transparent organic protective film, so the durability and appearance customers expect from a plated surface are still there while the environmental burden of the operation drops.
Because the film holds up, parts need fewer re-treatments and less material ends up as waste, which shows up as lower running cost rather than an inexpensive input that fails early. The same efficiency carries into passivation of stainless steel and into zinc-nickel plating work, so a shop can hold its finish standards without the extra spend that comes from reworking or reapplying. Ask for a cost comparison based on your own bath volume and current consumption.
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.
No. The film is transparent and formulated so it generally does not affect the original color of the passivation layer, so blue, white and color tones come through as they went in. If the job is matched to a physical approval sample, seal a trial piece from the same bath and compare it under the light your inspector uses before the run starts.
No. Passivation creates the protective conversion layer on the zinc; the seal goes on afterwards and protects that layer. Running the seal without a sound passivation underneath will not give you the color or the corrosion performance the finish was specified for.
Yes, with a separator fitted — the seal bath is not intended to run straight through a barrel without one. Immersion is short and at room temperature, so the step drops into the barrel line's existing cycle without changing its rhythm.
60–80 °C. The film forms as the water leaves, and that is also when the hardness and scratch resistance develop. A part that comes out under-dried feels tacky and will not hold up to handling or packing.
Seal and dry a trial piece from the same passivation batch and set it beside a part you would normally ship. The color should be unchanged, and once the film has dried the surface should feel harder and resist light scratching noticeably better. That trial also tells you which end of the 5–10% range suits your parts.
Talk to your sales contact about what your line needs. Concentration, packing format and how the product is supplied are handled case by case with our technical team, and they will confirm what is possible for your volumes before you place the order.
