The right finish for the service, applied to the specification.
A coating decides how a bolted joint ages: whether it rusts, whether it galls, and whether it comes apart at the next turnaround. Finishes are applied by outside coaters and platers to the ASTM, ISO and military specifications below, and coating thickness is reported on the certificate when the order asks for it.
Options · Pick by service
10 finishes we supply.
Low friction, easy break-out, the oil and gas standard
PTFE / fluoropolymer coating
A thin fluoropolymer film, usually blue or green, applied over a phosphate base. It gives a low, consistent coefficient of friction, so less torque is needed for the same bolt tension and the torque is set for the coated parts; it resists the atmosphere and chemical splash, and helps nuts come off at the next turnaround.
Specification
Xylan® 1424, 1070 or an equivalent fluoropolymer, in blue, green or other colours · to the customer's coating specification
Thickness
Typically 1 mil (25 µm) over phosphate; confirm the coating system and colour on the order
Best for
Studbolts and nuts for refinery, petrochemical, offshore and subsea flanges
Economical sacrificial protection for indoor and mild service
Zinc electroplating
A thin electroplated zinc layer with a clear, yellow or black chromate, or a chromium-free passivate. Zinc corrodes in preference to the steel underneath, so a scratch does not rust. It is the general-purpose finish for hardware that lives indoors or under cover.
Specification
ASTM F1941 for threaded fasteners (Fe/Zn 3, 5, 8 and 12 µm) and ASTM B633 for other parts (Fe/Zn 5 to 25 µm) · chromate or passivate type per the order
Thickness
Fe/Zn 5 (0.0002 in.) is the usual fastener class; heavier classes on request where the thread tolerance allows
Best for
Bolts, nuts and washers for mild atmospheres; parts above 39 HRC are baked for hydrogen embrittlement relief
Parts are dipped in molten zinc, which alloys with the steel to form a thick, tough coating with a long life outdoors. The coating is far heavier than electroplating, so nuts are tapped oversize after galvanizing to run on the coated bolt.
Specification
ASTM F2329 (fasteners) and ASTM A153 Class C · nuts overtapped per ASTM A563
Thickness
About 2 mil (50 µm) on fasteners over 3/8 in., less on smaller sizes; often thicker, depending on steel chemistry
Best for
Anchor bolts, U-bolts, stay rods and structural bolting for waterworks, transmission and outdoor plant (not F3125 Grade A490 or other 150 ksi bolts)
High corrosion resistance without hydrogen from plating
Zinc-flake coating
A sprayed or dipped coating of zinc and aluminium flakes in a binder, cured rather than plated. Because it is not electroplated, the coating process itself puts no hydrogen into the steel, which makes it a common choice for high-strength bolting that needs long salt-spray life.
Specification
ASTM F3393 · ISO 10683 · Geomet®, Delta Protekt® or equivalent systems
Thickness
Thickness and salt-spray life depend on the coating system; taken from the coating maker's data sheet
Best for
Property class 10.9 and 12.9 and Grade 8 bolting, wedge lock washers, wind and mobile-plant hardware
A manganese or zinc phosphate conversion coating that holds oil in its crystalline surface. It gives a dark grey to black finish, mild corrosion protection in storage and transit, and a good base for PTFE or paint.
Specification
DoD-P-16232 / MIL-DTL-16232 · ISO 9717
Thickness
Typically 5 to 10 µm (manganese) or 5 to 15 µm (zinc); the thread fit allows for it
Best for
A193 B7 studs with A194 2H nuts, and A320 L7 studs with A194 Grade 7 nuts, shipped plain, and as the base under PTFE
A chemical conversion of the steel surface to black iron oxide, sealed with oil or wax. It adds no thickness, so it suits precision parts and socket products, and gives a uniform dark appearance with light corrosion resistance indoors.
Specification
MIL-DTL-13924
Thickness
None; the finish is in the surface itself
Best for
Socket head cap screws, set screws, 12-point flange screws and nuts, and machined components
A thin silver layer plated onto the nut thread that acts as a solid lubricant. It stops austenitic and nickel-alloy nuts galling on their studs, including in high-temperature service.
Specification
ASTM B700
Thickness
As specified on the order; nuts may be tapped to suit
Best for
A194 Grade 8, 8M and nickel-alloy nuts for steam and high-temperature service
Uniform, hard and corrosion resistant, even inside threads
Electroless nickel
A nickel-phosphorus deposit laid down by chemical reduction rather than current, so it builds evenly on every surface, including thread roots and bores. It is harder than zinc and resists chemicals and wear.
Specification
ASTM B733
Thickness
As specified, by service condition (ASTM B733)
Best for
Valve and pump components, fittings and fasteners in chemical and marine atmospheres
A nitric or citric acid treatment that dissolves free iron left on the surface by tooling and lets the natural chromium-oxide film reform. It is not a coating; it is how a machined stainless part gets its full corrosion resistance back.
Specification
ASTM A967 · ASTM A380 (cleaning and descaling)
Thickness
None; a surface treatment
Best for
Machined 304, 316, 17-4 PH and duplex stainless parts
A soft, lubricious sacrificial coating with excellent salt-fog resistance and low galvanic action against aluminium. Its use is restricted by environmental rules, so we supply it only where a drawing or military specification still calls for it.
Specification
SAE AMS-QQ-P-416
Thickness
Class 1, 2 or 3 per the specification (0.0005, 0.0003 or 0.0002 in.)
Best for
Defence and aerospace ground-equipment hardware built to existing drawings
Where will the joint live, how strong is the bolt, and does it have to come apart again? Answer those and the finish usually picks itself.
Outdoors or buried: hot-dip galvanizing on carbon and low-alloy steel, but not on 150 ksi bolts such as F3125 Grade A490 or on parts of 40 HRC and above; stainless where the thread must stay fine.
Refinery, offshore, chemical plant: PTFE over phosphate on B7 / 2H and L7 / Grade 7 sets; it protects the thread and makes break-out easier.
High-strength bolting, over 39 HRC: zinc-flake. ASTM F1941 does not recommend electroplating these parts; where it is done, they must be baked.
Stainless and nickel alloys: passivate stainless parts, and silver-plate or lubricate the nuts to stop galling; no sacrificial coating is needed.
Indoor, cosmetic or precision: black oxide or clear zinc, which add little or no thickness to a toleranced thread.
Drawings that call for cadmium: we can still supply it, but ask whether zinc-nickel or zinc-flake will be accepted instead.
