Hot-dip galvanizing
Hot-dip galvanizing dips cleaned steel into molten zinc, which reacts with the iron to grow a thick coating of zinc-iron alloy layers under a skin of pure zinc. On a fastener it is about ten times as thick as zinc plating, so the nuts are tapped oversize to run on the galvanized bolt. Galvanizing is done by outside Houston galvanizers.
Clean, pickle, flux, dip, spin.
- STEP 01
Degrease
A hot alkali, mild acid or biological bath removes oil, grease and paint marks. Zinc will not react with dirty steel, so a missed spot comes out bare.
- STEP 02
Pickle
Dilute heated sulfuric or room-temperature hydrochloric acid strips scale and rust. High-strength parts are baked after pickling, or blasted and only flash-pickled, to limit the hydrogen they take in.
- STEP 03
Flux
A zinc ammonium chloride flux takes off the last oxide and keeps the clean steel from oxidizing again until it enters the zinc.
- STEP 04
Dip
The parts go into molten zinc of at least 98% purity at 820 to 860 °F (438 to 460 °C), usually for less than ten minutes, while the zinc and iron react.
- STEP 05
Spin and quench
Fasteners come out in a basket that is spun in a centrifuge to throw surplus zinc out of the threads, then quenched in water unless air cooling is ordered.
- STEP 06
Tap the nuts
Nuts are commonly galvanized as blanks and tapped oversize afterwards, then lubricated. The bolts keep their normal thread.
- Not for 150 ksi structural bolts such as ASTM F3125 Grade A490. F3125 lists no hot-dip coating for them; the only coating it lists as qualified is zinc-flake to ASTM F3393.
- Not for fasteners heat treated to 40 HRC or more, or case-hardened (ASTM F2329). From 33 HRC, the hydrogen risk has to be managed.
- Heat: zinc-coated bolting should stay below about 390 °F (210 °C) in service (ASTM A193, A194).
- Fine threads: A563 Table 5 covers coarse and 8-thread series nuts only. Other threads need an agreed overtap.
Finishes are applied by outside coaters and platers in Houston, to the specification on the order.
Thick zinc, oversize nuts.
A galvanized fastener over 3/8 in. carries about 0.002 in. (50 µm) of zinc, far too much for a standard thread fit. The bolt is threaded to normal class 2A size before galvanizing, and the nut is tapped oversize after it, by the allowance in ASTM A563 Table 5: 0.018 in. on the diameter for 1/2-13, 0.024 in. for 1-8, 0.027 in. for 1-1/2-6, and 0.050 in. from 1-3/4 to 4 in.
Cutting the nut thread larger leaves less thread in contact. A563 therefore proof-tests overtapped nuts after galvanizing and overtapping, and sets lower proof loads for some grades: a heavy hex DH nut is proof-loaded at 175 ksi plain but 150 ksi zinc-coated. With 8UN threads from 1-3/4 in., overtapped nuts may not develop the full strength of an anchor bolt near its minimum-material limits (ASTM F1554). The overtapped nut thread is left bare; the zinc on the bolt protects it.
Pickling puts hydrogen into the steel. ASTM F2329 flags a hydrogen embrittlement risk from a specified minimum hardness of 33 HRC, and does not allow hot-dip galvanizing of fasteners heat treated to 40 HRC or more, or of case-hardened ones. Galvanizing can also change the properties of heat-treated steel, so bolting standards retest the mechanical properties after it.
Designations and classes.
How to call it out.
- Order the bolts, nuts and washers galvanized by the same process, for example ASTM F3125 Grade A325 Type 1 heavy hex bolts, hot-dip galvanized to ASTM F2329, with ASTM A563 Grade DH heavy hex nuts, hot-dip zinc-coated, overtapped and lubricated, and ASTM F436 washers galvanized to F2329.
- For anchor bolts, give the length to be galvanized, measured from the exposed end (ASTM F1554).
- Galvanized ASTM A194 2H nuts used in place of A563 DH follow A563's coating, overtap and lubrication rules. They are marked 2HZN when supplementary requirement S8 is ordered.
- Ask for coating thickness on the certificate if you need it.
The tests behind it.
Coating thickness and salt spray
Coating thickness is measured with a magnetic or eddy-current gauge, or by X-ray fluorescence for thin platings, and checked against the class ordered. Salt-spray exposure checks corrosion protection on a sample from the lot when the specification calls for it.
Will the nut carry the bolt's full load?Proof load testing of nuts
The nut is threaded on a hardened mandrel and loaded to the proof stress for its grade and size. It passes if the threads neither strip nor break and it can be removed by hand afterwards; a wrench may start it, for up to half a turn.
Will a plated high-strength bolt crack in service?Hydrogen embrittlement relief and testing
High-strength steel can pick up hydrogen during pickling and plating and fail days later under load. Parts above 39 HRC are baked after plating to drive the hydrogen out, and a sustained-load test on fasteners from the lot proves the relief worked.
Does every dimension and thread meet the drawing?Thread gauging and dimensional inspection
Threads are checked with Go and No-Go ring and plug gauges, and pitch diameter is measured by the three-wire method or on a thread comparator. Heads, flats, lengths and radii are measured with calibrated instruments and, for complex parts, on the CMM.
Hot-dip galvanizing: common questions.
Can F3125 Grade A490 bolts be galvanized?
No. ASTM F3125 does not list hot-dip galvanizing for its 150 ksi grades; the coating it lists as qualified for them is zinc-flake to ASTM F3393.
Why are galvanized nuts tapped oversize?
The zinc on the bolt is about 0.002 in. (50 µm) thick, so the nut thread has to be cut larger to run on it. ASTM A563 Table 5 sets how much, and the nut is proof-tested after overtapping.
F2329 or A153?
F2329 is written for fasteners: it adds water quenching, an adhesion test on every batch and an embrittlement test. A153 Classes C and D are the older hardware equivalents and still appear on drawings.
Will the bare nut threads rust?
The overtapped nut thread is left uncoated, but once the nut is on, the zinc on the bolt protects it.
Compare with.
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.
Economical sacrificial protection for indoor and mild serviceZinc 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.
Low friction, easy break-out, the oil and gas standardPTFE / 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.