Chain Coatings and Surface Treatments: Zinc, Nickel, Dacromet and Lubricant Films for Corrosion-Prone Applications

A carbon steel roller chain carries load through hardened pins and bushes rated to tensile strengths of 30 kN to over 150 kN depending on pitch and series. That same steel, left bare, will show red rust inside 72 hours of exposure to a coastal atmosphere where the annual chloride deposition can exceed 60 mg per square metre per day. For buyers sourcing chains for washdown lines, outdoor conveyors, marine deck equipment or chemical plant elevators, the base material is only half the story. The coating or surface treatment decides whether the chain survives 48 hours or 1,000 hours plus before the first corrosion pit appears. This article compares the commercial surface treatments available on carbon steel industrial chain, gives the salt spray numbers buyers can quote in a specification, and explains where each process breaks down in real service.

Why Carbon Steel Chain Still Needs a Coating

Stainless chains solve corrosion by changing the alloy, but they cost 2.5 to 4 times more than a carbon steel equivalent and lose roughly 20 percent of their working load limit in the common 304 and 316 grades. Most buyers therefore stay on carbon steel and protect the surface instead. A bare 40Mn or 45Mn pin treated only with rust-preventive oil will fail a neutral salt spray test at about 24 to 48 hours, and the oil film is wiped off by the first abrasive contact in the drive. A proper conversion coating or metallic layer moves that failure point out to 240, 500 or even 1,000 hours plus. The choice is a procurement decision, not an engineering afterthought, because it changes the unit price by 8 to 35 percent and changes the minimum order quantity a supplier will accept.

Zinc Plating — White, Yellow and Black Variants

Electroplated zinc is the default low-cost option on catalogue chains. A typical deposit runs 5 to 12 μm thick and the appearance is controlled by the final passivation film. White zinc uses a clear trivalent passivation and reaches a neutral salt spray life of about 48 to 96 hours to white corrosion. Yellow, now almost always zinc plating with trivalent passivation, pushes that to 96 to 240 hours in most production batches. Black zinc, achieved with a black trivalent passivation or a thin black organic topcoat, sits at 96 to 200 hours and is chosen mainly for appearance on visible drives and display mechanisms.

The cost premium over bare oiled chain is small, normally 8 to 15 percent, which is why zinc plating is the most common finish on standard roller chains shipped to distributors. It works for indoor, dry and mildly humid duty. It does not survive continuous washdown, outdoor coastal exposure or contact with acidic process fluids. Another limit is thickness control: plating builds up on edges and in the link clearance, so pins must be plated before assembly or the joint articulation is compromised. For assembled chains, rack plating of sub-components is the standard method, and that adds handling cost plus a small scrap rate on the softer plates.

Nickel Plating for Higher Corrosion and Wear Resistance

Nickel electroplating delivers both corrosion resistance and a harder, lower-friction surface. A 10 to 25 μm nickel layer typically survives 240 to 500 hours in the ASTM B117 cabinet and the deposited hardness of 400 to 550 HV reduces galling on pins and bushes. Buyers specify nickel where the chain runs in food and pharmaceutical washdown but where stainless is rejected on cost, or where a decorative bright finish is required on exhibition equipment. The penalty is price: nickel plating adds 25 to 45 percent to the chain cost and extends lead time by one to two weeks because of the slower plating cycle and stricter bath control plus the mandatory post-bake.

Nickel carries the same hydrogen embrittlement exposure as zinc because it is an aqueous electroprocess, so it is unsuitable for through-hardened high-tensile chain above about 1,000 MPa without a documented relief bake. Where buyers need both the bright finish and high strength, suppliers usually switch to a mechanical or thin-film alternative instead of thick nickel, or they drop to a lower-strength grade that sits safely under the 1,000 MPa threshold.

Dacromet and Geomet — The Salt Spray Champions

When the environment is genuinely hostile, water-based zinc and aluminium flake coatings dominate. Dacromet and the related Geomet process deposit a 6 to 12 μm film of zinc and aluminium flakes bound in an inorganic matrix, cured at 200 to 320 °C. They routinely pass 500 to 1,000 hours of neutral salt spray without red rust, and thick-build variants exceed 1,000 hours in qualified batches. The flake structure means there are no micro-cracks and no pathway for chloride to reach the substrate, which is why these coatings are standard on automotive underbody and agricultural chains exposed to road salt and slurry.

Buyers should note that Dacromet 500 coating system is a specific formulation with published 500-hour-plus data, and Geomet 321 or 500 series are the equivalents from a different supplier. The two are not interchangeable in a spec, so write the trade name if your application was qualified against it. Cost is 30 to 55 percent above bare chain, and the curing oven size limits the maximum single chain length that can be coated in one pass, which can force shorter shipped lengths on very long conveyors. Dacromet and Geomet also carry no hydrogen risk because there is no aqueous electrolysis, which is the main reason they are preferred on high-strength chains.

Nitriding and Other Diffusion Treatments

Gas or salt-bath nitriding diffuses nitrogen into the surface of the steel to form a compound layer roughly 8 to 20 μm deep with a hardness of 600 to 900 HV. Unlike a plated layer, it cannot flake off because it is the same material as the substrate. Corrosion life is moderate, around 120 to 300 hours, but wear and scuff resistance are excellent, so nitriding is chosen for dry-running or low-lubrication chains in dusty plants rather than for pure corrosion duty. The process temperature of 480 to 580 °C can cause slight dimensional growth and may temper higher-hardness pins, so it is applied to selected components rather than the whole assembled chain. Cost sits between zinc and nickel, about 18 to 30 percent over bare, and lead time grows by roughly one week.

Pre-Applied Lubricant Films

A separate category is the factory-applied dry or semi-dry film that sits on top of, or instead of, a conversion coating. These are wax, PTFE, molybdenum disulphide or soap-based layers that protect the chain in storage and during run-in. A typical preload lubrication film adds 3 to 8 μm and extends the rust-proof shelf life to 12 to 24 months in a warehouse, while also reducing the initial wear rate by 15 to 40 percent versus an oiled-only chain. For food plants the film must be NSF H1 registered, which excludes most standard rust preventives and is a common reason a quoted chain is rejected at incoming inspection. These films are not a salt spray solution on their own; they are paired with zinc or Dacromet beneath to give both storage protection and cabinet life.

Salt Spray Performance — Reading the Numbers

The single most abused number in chain procurement is the salt spray hour rating. The reference is the ASTM B117 salt spray test, a neutral 5 percent sodium chloride fog at 35 °C. It is a comparative corrosion-acceleration tool, not a field-life predictor, yet buyers use it as a pass-fail gate. A realistic mapping for carbon steel chains is:

White zinc, trivalent: 48 to 96 hours to white rust, red rust around 96 to 150 hours.
Yellow zinc, trivalent: 96 to 240 hours to first red rust.
Black zinc: 96 to 200 hours to red rust.
Nickel 10 to 25 μm: 240 to 500 hours to red rust.
Dacromet or Geomet 6 to 12 μm: 500 to 1,000+ hours to red rust.
Nitrided surface: 120 to 300 hours, with superior abrasive wear life.

Field experience shows that each 100 hours of ASTM B117 typically corresponds to roughly 1 to 3 months of sheltered outdoor service or 2 to 6 weeks of direct coastal exposure, depending on humidity and pollutant load. Buyers who specify 1,000 hours without understanding this often over-pay for a coating their environment does not need, while buyers who quote 48 hours for a port crane chain buy a failure.

Coating Thickness, Wear Balance and Cost Gradient

Thickness is a trade. Too thin and the salt spray number collapses; too thick and the coating spalls in the articulating joint or binds the chain. A plated layer above about 15 μm on a roller chain joint starts to fill the plate hole clearance and raises the breakaway torque. The practical window is 8 to 12 micrometres for flake coatings and 5 to 12 μm for electroplate, which is why suppliers quote those bands rather than “as thick as possible”. The cost gradient from cheapest to most expensive is roughly oiled bare at 0 percent, white zinc at plus 8 to 12 percent, yellow zinc at plus 10 to 15 percent, black zinc at plus 12 to 18 percent, nitrided at plus 18 to 30 percent, nickel at plus 25 to 45 percent, and Dacromet or Geomet at plus 30 to 55 percent. MOQ follows the same slope: a supplier may accept 50 metres of zinc-plated chain but will demand 300 to 1,000 metres of Dacromet work to amortise the oven set-up and the slurry batch.

Hydrogen Embrittlement Risk on High-Strength Chains

Aqueous electroplating absorbs atomic hydrogen into the steel lattice, and on through-hardened parts above about 1,000 MPa tensile the result can be delayed fracture under load. This is the hydrogen embrittlement risk that rules out thick zinc or nickel on high-strength leaf chains, lifting chains and certain engineered drive chains. The mitigation is a post-plate bake at 190 to 230 °C for 4 to 24 hours, which the supplier must document on the certificate. Many buyers therefore specify Dacromet or Geomet, which are applied from water-based slurry without electrolysis and carry no embrittlement risk, or they accept a lower-strength grade to keep the electroplate option open. Writing “baked after plating, per ASTM F1941 or ISO 9587″ in the spec closes the loop and gives incoming inspection a checkable line.

Food-Grade Constraints and Coating Selection

In meat, dairy, beverage and pharmaceutical plants the chain contacts surfaces that may touch product, so the coating and any film must be food-safe. Zinc and nickel are not accepted in direct product zones without an NSF H1 lubricant barrier, and epoxy or organic topcoats must carry the right registration. Dacromet formulations exist in food-contact-approved versions, but the buyer has to name the exact approval, because a standard automotive Dacromet is not automatically food-grade. The washdown chemical matters too: caustic cleaners at pH above 12 attack zinc within days, while nickel and flake aluminium coatings tolerate them far better. Specifying the cleaner pH and temperature is as important as the coating name, and a chain that survives the salt spray cabinet can still be eaten away by the wrong daily washdown chemical.

Environmental Selection Guide — Humid, Coastal, Chemical

Matching the coating to the duty avoids both premature failure and wasted spend. For indoor dry or lightly humid plants with occasional condensation, white or yellow zinc at 96 to 240 hours is sufficient and cheapest. For humid tropical warehouses and unheated outdoor frames, move to yellow zinc at the top of its range or nitride for wear, budget 240 to 500 hours. For coastal ports, desalination plants and marine decks where chloride is constant, specify Dacromet or Geomet at 500 to 1,000+ hours and accept the 30 to 55 percent premium as the price of a two-to-three-year service interval. For chemical and fertilizer handling, the process fluid decides: acid mist favours nickel or flake aluminium, alkali favours nickel over zinc, and abrasive solids favour nitride for wear even if corrosion life is only moderate.

Writing Coating Type and Salt Spray Grade into the PO Spec

A purchase order that says only “galvanised chain” is unenforceable and invites the cheapest possible interpretation. Buyers should state the process, the thickness band, the test standard and the acceptance hour, for example: “Surface treatment: electroplated zinc, yellow trivalent passivation, 8 to 12 μm minimum local thickness, 240 hours neutral salt spray to red rust per ASTM B117, certificate required.” For flake coatings write “Dacromet 500 or approved equal, 6 to 10 μm, 720 hours minimum per ASTM B117, no red rust.” Include the embrittlement clause for high-strength grades, the food approval number for hygienic duty, and the pack shelf-life expectation if the chain will sit in storage before installation. A precise spec lets the supplier quote a firm price and lets your incoming inspection reject non-conforming lots instead of arguing about whether the chain “looks rusty” after two weeks on the dock.

What Happens After Coating Damage

No coating is permanent. Once a zinc or nickel layer is scratched through to the steel, corrosion starts at that point and undercutting spreads beneath the surrounding film, so a small nick can cost 20 to 40 percent of the rated salt spray life locally. Flake coatings such as Dacromet resist undercutting better because there is no continuous metallic pathway, but they still expose steel at a deep gouge. Nitriding, being integral to the substrate, shows the least performance drop after surface scoring because the protection is the steel itself. The practical rule for maintenance buyers is to re-apply a compatible preventive film at the first sign of bright steel and to keep a spare length in safety stock, because a coated chain that has lost its layer will then degrade on the same clock as a bare chain and take the line down with it.

Tip: Always request the salt spray test certificate (ASTM B117) and the coating thickness report (μm) from your supplier before releasing the purchase order, because a verbal “corrosion resistant” claim is not a measurable specification.

FAQ — Coating and Surface Treatment Questions from Buyers

Is zinc plating enough for an outdoor conveyor near the sea?

No. Standard yellow zinc gives 96 to 240 hours in the ASTM B117 cabinet, which translates to only weeks of direct coastal service. For ports and marine decks specify Dacromet or Geomet at 500 to 1,000+ hours, or expect to replace the chain every season and absorb the unplanned downtime that comes with it.

Why will a supplier refuse to plate my high-strength lifting chain?

Through-hardened chains above about 1,000 MPa tensile absorb hydrogen during aqueous plating and can fail by delayed fracture under load. The supplier either post-bakes per ASTM F1941, switches you to a flake coating with no embrittlement risk, or declines the electroplate entirely for liability reasons rather than ship a chain that may snap.

Does a thicker Dacromet coating always last longer?

Up to a point. The usual 6 to 12 μm band already reaches 500 to 1,000+ hours. Going thicker adds marginal salt spray life but raises curing cost and can bridge the joint clearance on small-pitch chains, so most suppliers cap the build rather than maximise it and balance corrosion life against free articulation.

Can I use a standard zinc chain in a food washdown line?

Only with an NSF H1 barrier film and never in direct product contact zones without food-grade approval. Alkaline cleaners at pH above 12 will strip zinc in days, so nickel or food-approved flake coatings are the safer long-term choice despite the higher up-front cost and longer lead time.

How do I prove the coating meets the spec after delivery?

Require the salt spray certificate (ASTM B117, hours to red rust) and the coating thickness report (μm, per ISO 2178 or ASTM B487) with the shipment. Without those two documents a salt spray claim is unverifiable and should be rejected at incoming inspection before the chain enters the storeroom.

Post time: Sep-28-2026

Chain Coatings and Surface Treatments: Zinc, Nickel, Dacromet and Lubricant Films for Corrosion-Prone Applications