Mainstream Surface Treatments and Corrosion Classes

Surface treatment on a fastener serves three purposes: corrosion protection, friction coefficient control, and appearance. The choice depends on the required service life, the strength class of the bolt (because high-strength parts are hydrogen-sensitive), the assembly method, and the budget. Below is a comparison of the most common treatments with typical neutral salt spray (NSS) performance per GB/T 10125.

Zinc Plating: Basic but Easy to Get Wrong

Electrolytic zinc plating is the most common surface treatment for carbon steel fasteners. It deposits a thin zinc layer on the bolt that provides both barrier protection and sacrificial (galvanic) protection: the zinc corrodes preferentially, protecting the steel underneath. With trivalent chromium passivation, a zinc-plated bolt reaches 72-200 hours of neutral salt spray before red rust appears.

The catch is hydrogen embrittlement. The acid cleaning and electroplating process releases hydrogen atoms that diffuse into the steel. For bolts of 10.9 class and above (hardness above 320 HV), this hydrogen can cause delayed fracture days or weeks after plating. The standard mitigation is a bake-out within 4 hours of plating at 190-220 C for 4-8 hours. If the plater skips the bake or delays it, high-strength bolts can fail in service. For 12.9 class bolts, many engineers avoid electrolytic plating entirely and specify a non-electrolytic coating such as dacromet.

Dacromet / Zinc-Aluminum: No Hydrogen Risk for High-Strength Parts

Dacromet (the original trade name; generic terms are zinc-aluminum flake coating, Geomet, Delta-Protekt) is a non-electrolytic coating. The fastener is dipped or spun in a water-based slurry of zinc and aluminum flakes, then cured in an oven at about 300 C. Because there is no electroplating current and no acid pickling, almost no hydrogen is introduced. This makes dacromet the preferred coating for 10.9 and 12.9 class bolts where hydrogen embrittlement is a concern.

Corrosion performance is excellent: 500-1500 hours of neutral salt spray before red rust, depending on coating thickness. The coating is also thin (typically 8-15 µm), so it does not significantly change thread dimensions, and it has good high-temperature resistance. The downsides are cost (higher than zinc plating) and a limited temperature ceiling during curing, which means it cannot be applied to parts that already have heat-treated lubricants that would degrade at 300 C.

Zinc-Nick Plating: NEV Industry Standard

Zinc-nick alloy plating deposits a zinc layer containing 12-15% nickel. The nickel dramatically improves corrosion resistance, reaching 500-1000 hours of neutral salt spray with the correct passivation. It has become the standard for new energy vehicle fasteners because it combines good corrosion resistance with a controlled friction coefficient and relatively low cost compared to dacromet. The battery pack, electric drive, and body fasteners on most Chinese NEVs now use zinc-nick plating.

Like all electrolytic plating, zinc-nick carries hydrogen embrittlement risk on high-strength bolts. The industry standard requires bake-out within 2 hours of plating for 10.9 and 12.9 class bolts. A qualified supplier will have a documented bake process and can provide hydrogen embrittlement test reports per GB/T 3098.17.

Black Oxide and Stainless Passivation

Black oxide is a chemical conversion coating that turns the steel surface black. It provides almost no corrosion protection on its own (4-12 hours salt spray) and relies on oil impregnation for temporary protection. It is used where appearance matters more than corrosion: precision machine tools, firearms, and equipment covers that live indoors. It is cheap, does not change dimensions, and introduces no hydrogen risk.

Stainless passivation is a nitric acid or citric acid treatment that removes free iron from the stainless surface and strengthens the passive chromium oxide film. It does not add a coating; it activates the material's inherent corrosion resistance. Per GB/T 3098.6, passivation is a required delivery process for stainless fasteners rather than an optional extra. Passivated A2-70 bolts can exceed 500 hours of neutral salt spray, and A4-70 can exceed 1000 hours.

Hydrogen Embrittlement Risk and Selection

The single most important selection rule is this: bolts harder than 320 HV (roughly 10.9 class and above) must not receive electrolytic plating without a documented bake-out, and for critical safety parts, specify a non-electrolytic coating such as dacromet. For 4.8 to 8.8 class bolts, hydrogen risk is low and zinc plating or black oxide is fine without bake-out. For outdoor or under-hood exposure, choose zinc-nick or dacromet. For indoor dry use, black oxide with oil is adequate.

Specifying surface treatment on the drawing. A surface treatment callout should include the coating type, minimum salt spray hours, and any hydrogen bake-out requirement for high-strength parts. For example, a 10.9 class body fastener might be specified as "Zinc-nick plating, 8-12 micrometer, trivalent passivate, 500 hours NSS minimum, bake-out within 2 hours of plating per GB/T 3098.17." Vague callouts like "zinc plated" leave the coating thickness, passivation, and bake-out to the supplier's discretion, which leads to inconsistency between lots. Be specific about the environment the part lives in so the supplier can match the coating to the corrosion demand.

Hydrogen embrittlement bake-out details. For high-strength plated fasteners, the bake-out is not optional. The standard practice is to bake within 2 hours of plating completion at 190-220 C for 4 hours minimum (8 hours preferred for 12.9 class). The clock starts when the plating current stops; delays in racking and transferring to the oven let hydrogen diffuse deeper into the steel. Reputable platers track the time from plating to bake and provide records. If a supplier cannot document the bake-out time and temperature, do not accept the parts for safety-critical applications. The cost of a hydrogen embrittlement failure in the field dwarfs the cost of a properly documented bake-out process.

Coating thickness and thread fit. Every coating adds thickness to the thread profile. Zinc plating adds 5-12 micrometers; zinc-nick adds 5-10; dacromet adds 8-15. On a standard 6g external thread, this thickness is usually acceptable, but on precision fits or when the thread is at the tight end of tolerance, plating can push it over. This is why plated bolts are specified 6g rather than 6h: the 6g external thread starts slightly undersized, leaving room for the plating. If you have a plating thickness specification, verify that the finished thread still passes the NO-GO gauge after coating. A post-plating thread gauge check catches coating-thickness issues before they reach the assembly line.

Specifying surface treatment on the drawing. A surface treatment callout should include the coating type, minimum salt spray hours, and any hydrogen bake-out requirement for high-strength parts. For example, a 10.9 class body fastener might be specified as "Zinc-nick plating, 8-12 micrometer, trivalent passivate, 500 hours NSS minimum, bake-out within 2 hours of plating per GB/T 3098.17." Vague callouts like "zinc plated" leave the coating thickness, passivation, and bake-out to the supplier's discretion, which leads to inconsistency between lots. Be specific about the environment the part lives in so the supplier can match the coating to the corrosion demand.

Hydrogen embrittlement bake-out details. For high-strength plated fasteners, the bake-out is not optional. The standard practice is to bake within 2 hours of plating completion at 190-220 C for 4 hours minimum (8 hours preferred for 12.9 class). The clock starts when the plating current stops; delays in racking and transferring to the oven let hydrogen diffuse deeper into the steel. Reputable platers track the time from plating to bake and provide records. If a supplier cannot document the bake-out time and temperature, do not accept the parts for safety-critical applications. The cost of a hydrogen embrittlement failure in the field dwarfs the cost of a properly documented bake-out process.

Coating thickness and thread fit. Every coating adds thickness to the thread profile. Zinc plating adds 5-12 micrometers; zinc-nick adds 5-10; dacromet adds 8-15. On a standard 6g external thread, this thickness is usually acceptable, but on precision fits or when the thread is at the tight end of tolerance, plating can push it over. This is why plated bolts are specified 6g rather than 6h: the 6g external thread starts slightly undersized, leaving room for the plating. If you have a plating thickness specification, verify that the finished thread still passes the NO-GO gauge after coating. A post-plating thread gauge check catches coating-thickness issues before they reach the assembly line.