Types of Surface Treatment and Plating
Short answer
- Surface treatment sets a fastener's corrosion resistance, dimensional fit, and torque-tension behavior, so each coating must match the base metal and the application.
- Electroplating introduces hydrogen embrittlement risk in high-strength steel, hot-dip galvanizing at 45 to 85 microns alters thread fit, and zinc flake coatings at 8 to 20 microns generate no hydrogen at all.
- Passivation, anodizing, black oxide, phosphate, and PTFE coatings each change corrosion resistance, friction, or dimensions differently, with typical thicknesses running from under 1 micron up to 60 microns.
Specifying a fastener or machined component requires more than just choosing the base metal; the surface treatment dictates corrosion resistance, dimensional fit, and torque-tension relationships. Applying the wrong coating can turn a precision bolt into a failure point. For high-strength fasteners and hardware, plating introduces hydrogen embrittlement risks. For structural connections, galvanizing alters thread geometry. Here is a breakdown of the common industrial surface treatments, their typical thicknesses, and their physical effects on the finished part.
What does electroplating do to fasteners?
Electroplating uses an electrolytic cell to deposit a metal layer onto a conductive substrate. The part acts as the cathode, attracting metal ions from the anode and the electrolyte solution. While it provides excellent corrosion resistance and a uniform appearance, the process inherently introduces atomic hydrogen into the steel substrate.
- Zinc and Zinc-Alloy: The most common plating for steel fasteners. Pure zinc offers decent cathodic protection (it sacrificially corrodes to protect the underlying steel). Zinc-nickel or zinc-cobalt alloys offer significantly higher corrosion resistance. Typical thickness ranges from 5 to 15 microns (0.0002 to 0.0006 inches).
- Nickel: Used for chemical resistance, wear resistance, and aesthetic finish. It provides a hard, ductile barrier. Typical thickness is 10 to 25 microns.
- Chrome: Hard chrome plating is used for wear resistance on hydraulic rods and surfaces, built up to 25 microns or more. Decorative chrome is extremely thin, often less than 1 micron over a nickel base.
- Tin: Used for electrical conductivity and solderability. It is soft and prevents galling. Typical thickness is 2 to 10 microns.
Hydrogen Embrittlement Risk: During acid cleaning and the electroplating process, atomic hydrogen diffuses into the steel. In high-strength steels (typically above 39 HRC or 180 ksi tensile strength), the trapped hydrogen migrates to grain boundaries under stress, causing sudden, brittle catastrophic fracture. This is a critical safety issue. To remedy this, plated high-strength fasteners must undergo a bake-out process, typically at 190°C to 220°C (375°F to 425°F) for 8 to 24 hours immediately after plating, to drive the hydrogen out of the steel.
How does hot-dip galvanizing affect thread fit?
Hot-dip galvanizing involves immersing the steel part in a bath of molten zinc at around 450°C (840°F). The zinc reacts with the steel to form a metallurgical bond of zinc-iron alloy layers topped with pure zinc. It offers thick, heavy-duty cathodic protection for outdoor and marine environments. Typical coating thickness is 45 to 85 microns (1.5 to 3 mils), significantly thicker than electroplating.
The primary mechanical consequence of hot-dip galvanizing is the alteration of thread fit. The thick coating builds up on the flanks and roots of the threads. If internal and external threads are manufactured to standard 2A/2B or 6g/6H tolerances, the zinc buildup will cause binding or prevent assembly entirely. To accommodate this, external threads must be manufactured undersized, typically by removing material equal to the maximum galvanizing thickness, leaving a clearance gap that the zinc fills. The process also leaves a rougher surface, increasing the friction coefficient. When specifying structural bolts, you must account for this by using compatible adhesives, sealants and chemicals or lubricants to achieve the required clamp load without galling.
Why use zinc flake coatings on high-strength bolts?
Zinc flake coatings, sometimes applied mechanically, are non-electrolytic systems. They consist of zinc and aluminum flakes suspended in an inorganic binder, applied via dip-spin or spray, and cured by baking. Because no electricity is used, there is no hydrogen generation during the coating process, eliminating the hydrogen embrittlement risk entirely. This makes them ideal for high-strength fasteners, particularly Grade 10.9 and 12.9 class bolts.
Typical thickness is 8 to 20 microns. The coating is extremely thin and uniform, meaning it does not significantly alter thread dimensions. Standard threads can usually be used without special undersizing. The friction coefficient is relatively low and can be tightly controlled by the inclusion of topcoats, ensuring predictable torque-tension behavior.
What does passivating stainless steel actually do?
Passivation is not a coating; it is a chemical cleaning process that removes free iron and other surface contaminants from stainless steel. The part is immersed in an acid solution, typically nitric or citric acid. This dissolves the free iron particles left on the surface from machining, drilling, or forming tools. Once the free iron is removed, the chromium in the alloy naturally reacts with oxygen to form a thin, invisible chromium oxide layer.
Typical thickness of the passive layer is negligible, measuring only a few angstroms. It does not affect thread fit or dimensional tolerances. It does, however, significantly improve the corrosion resistance of the stainless part by preventing surface rusting and pitting. Passivation is standard practice for enclosure hardware and accessories where stainless steel must maintain both structural integrity and a clean, rust-free appearance in corrosive environments.
How does anodizing change aluminum parts?
Anodizing is an electrolytic passivation process unique to aluminum. Unlike plating, which adds material to the surface, anodizing converts the aluminum surface itself into a durable, corrosion-resistant aluminum oxide layer. The part is the anode in the electrolytic cell, hence the name.
- Type II (Sulfuric Acid Anodize): The standard anodizing process. It creates a porous oxide layer that can be dyed in various colors before being sealed. Typical thickness is 10 to 20 microns. It provides good corrosion and moderate wear resistance.
- Type III (Hardcoat Anodize): Uses a colder, higher-current process to create a much thicker, denser oxide layer. Typical thickness is 25 to 60 microns. It offers exceptional wear and abrasion resistance, approaching the hardness of a hardened tool steel.
The critical dimensional consideration for anodizing is growth. Because the oxide layer forms by converting the base aluminum, the layer grows both outward and inward. Roughly half of the coating thickness adds to the overall dimension of the part, while the other half penetrates into the substrate. A 50-micron hardcoat adds approximately 25 microns to the external dimension. For precision T-slot profiles and extrusions, this dimensional growth must be accounted for in the machining tolerances, or mating parts may not fit. The anodized surface is also an electrical insulator, meaning grounding points must be masked or machined post-anodize.
What do black oxide and phosphate coatings do?
Conversion coatings alter the surface chemistry of the metal without adding a substantial layer of material.
- Black Oxide: A chemical conversion coating for steel, stainless, and copper alloys. It produces a black, magnetite (Fe3O4) layer on steel. Typical thickness is less than 1 micron. It provides almost no dimensional change, making it ideal for precision parts and tight-tolerance assemblies. However, black oxide offers minimal corrosion protection on its own; it is highly porous and must be heavily oiled to provide any barrier to moisture. Without the oil, it rusts rapidly.
- Phosphate: Typically manganese or zinc phosphate. The chemical reaction creates a crystalline layer on the steel surface. Manganese phosphate is heavily used in automotive and machinery applications for its excellent oil-retention properties and anti-galling characteristics. Zinc phosphate is often used as a base for paint or rubber-to-metal bonding. Typical thickness is 5 to 20 microns. It adds slightly to dimensions but primarily changes the surface texture, increasing friction and providing a break-in layer for moving parts.
What do PTFE dry-film coatings do for fasteners?
Fluoropolymer coatings like PTFE (Teflon) and other dry-film lubricants are applied as a liquid or powder and then cured. They bond to the surface mechanically or through a primer system. The primary function is to drastically reduce the coefficient of friction, control torque-tension relationships, and prevent galling.
Typical thickness is 15 to 30 microns. PTFE coatings are soft and can wear off under high contact stresses, but in the threads of a fastener, they provide a consistent, low-friction surface that allows a high clamp load to be achieved with a relatively low input torque. They also offer moderate chemical and corrosion resistance. Because the coating is thin and uniform, it generally does not require special thread undersizing, though the friction coefficient must be factored into torque calculations.
If you need to source specific coated fasteners, cross-reference plating specifications, or find compatible hardware for your maintenance applications, our team can help identify and quote the correct components for your requirements.
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Why does electroplating cause hydrogen embrittlement in high-strength fasteners?
During acid cleaning and electroplating, atomic hydrogen diffuses into the steel. In high-strength steels, typically above 39 HRC or 180 ksi tensile strength, the trapped hydrogen migrates to grain boundaries under stress and causes sudden, brittle fracture. Plated high-strength fasteners must be baked at 190°C to 220°C for 8 to 24 hours immediately after plating to drive the hydrogen out of the steel.
How does hot-dip galvanizing affect thread fit?
Hot-dip galvanizing builds a thick coating of 45 to 85 microns on thread flanks and roots, which causes binding or prevents assembly if threads follow standard 2A/2B or 6g/6H tolerances. To accommodate it, external threads must be manufactured undersized by removing material equal to the maximum galvanizing thickness, leaving a clearance gap that the zinc fills. The rougher surface also raises the friction coefficient.
Which coating avoids hydrogen embrittlement on Grade 10.9 and 12.9 bolts?
Zinc flake coatings avoid it. They are non-electrolytic systems of zinc and aluminum flakes in an inorganic binder, applied by dip-spin or spray and cured by baking. Because no electricity is used, no hydrogen is generated, which makes them ideal for Grade 10.9 and 12.9 bolts. Typical thickness is 8 to 20 microns, so thread dimensions are not significantly altered.
How much does anodizing change part dimensions?
Anodizing converts the aluminum surface into oxide, so the layer grows both outward and inward. Roughly half of the coating thickness adds to the overall dimension, while the other half penetrates the substrate. A 50-micron hardcoat adds approximately 25 microns to the external dimension. This growth must be accounted for in machining tolerances, and the anodized surface is an electrical insulator, so grounding points must be masked.