Industries we serve

Construction & Structural

Anchors, structural bolts and hardware with durable corrosion protection for outdoor use.

Construction fasteners are rarely the strongest parts on a site — they are the most exposed ones. Anchor bolts, structural bolts, nuts and washers sit outdoors through a Thai monsoon, often in contact with galvanised purlins, aluminium cladding or wet concrete. Specification here is driven by corrosion protection and by dimensional fit after coating far more than by hardness, and the coating decision is a design decision, not a purchasing one.

Typical specification bands in this sector

Typical specification bands in this sector
Property class	4.6 / 4.8 / 8.8
Hardness	22–32 HRC (class 8.8, ISO 898-1)
Materials	SS400 · S45C · SCM435 · SWCH10B21
Coating system	Zn + Cr3+ with sealer / top-coat
Coating thickness	8–25 µm Zn (ASTM B633 SC2–SC4)
Salt spray (NSS)	96–480 h NSS
De-embrittlement bake	only at ≥ 320 HV or class ≥ 10.9
Inspection methods	ISO 3497 · ISO 9227 · ISO 1502 thread gauge

Construction specifications vary enormously between a covered warehouse and a coastal structure. These bands describe what is commonly written; the coating that actually protects your part follows from the exposure the designer assumed.

Standards commonly cited

ISO 898-1 ASTM B633 ISO 2081 ISO 4042 ISO 9227 ISO 3497 All standards →

Steel grades commonly specified

S45C SWCH10B21 SCM435 All steel grades →

How fasteners actually fail here

1Galvanic corrosion at mixed-metal joints

A steel fastener bolted through aluminium cladding, a stainless bracket or a hot-dip galvanised purlin forms a bimetallic couple, and the less noble metal corrodes first. Because the fastener is the small-area member, it corrodes fastest. Electroplated zinc is sacrificial and protects the steel underneath, but once the zinc is consumed the joint has no reserve left. Isolation washers, a thicker coating or a matched material are design answers; the finisher can only supply what the drawing specifies.

2Outdoor exposure — white corrosion first, then red rust

Zinc corrodes before the steel does, as white powdery zinc oxide and carbonate. That is cosmetic in itself but it signals the coating is being consumed; red rust means the steel underneath is exposed. Packing still-damp plated parts in sealed bags or stacking them wet is the classic cause of white corrosion that appears long before the parts ever reach the site.

3Thread fit lost after coating

Zinc deposits on all thread flanks, so the effective pitch diameter grows by roughly four times the local coating thickness. Nuts that gauged correctly bare will not run on after a heavy coating unless the allowance was designed in. ISO 4042 covers this and the usual answer is to plate the bolt and tap the nut oversize — decided before the lot is plated, not after it fails the gauge.

4Corrosion at the embedment line on anchors

Cast-in and post-installed anchors carry tension into concrete. Moisture and chlorides concentrate at the concrete-to-air interface, and the section corrodes exactly where the tensile load is highest. The anchor's design capacity belongs to the structural engineer; the coating specification is what keeps the section intact long enough for that capacity to mean something.

Defect guides that matter in this sector

White rust on zinc-plated partsWhite powdery zinc corrosion in storage and transit: passivation, drying, sealer and packaging faults that cause it, and how to judge it against a salt-spray acceptance criterion.Early red rust on plated fastenersRed rust well before the specified salt-spray hours: thin coating in thread roots and recesses, damaged passivation, handling damage and test-plan mismatches, with the checks that find the real cause.Thread gauge failure after platingWhy plated screws stop accepting the GO gauge: coating thickness lands in the thread allowance. How ISO 4042, ISO 965-1 and ISO 1502 handle it, and what to specify before production.Poor phosphate adhesion — coating rubs off, paint liftsPhosphate that wipes off, coarse powdery crystals and paint peeling from a phosphated fastener: cleaning, activation, bath control and post-treatment causes, with the practical checks that identify each.

All defect guides →

Documentation a buyer in this sector should receive

A mill / heat-lot certificate for the bar or wire, supplied by the fastener manufacturer. The coating report references the heat number rather than restating the chemistry.
A coating thickness report with the method and the measurement location, per ISO 3497 or magnetic induction, stated as a minimum local thickness rather than an average — a project inspector measures the thinnest point, not the mean.
A salt-spray report to ISO 9227 / ASTM B117 where the project specification names hours, stating hours, inspection interval, whether the criterion is white corrosion or red rust, and the significant surfaces.
Where the specification names a coating standard, quote the classification: an ASTM B633 service condition (SC1–SC4) or an ISO 2081 designation carries a thickness, not just the words "zinc plated".
Lot traceability from the delivered box back to the plating run, so a batch can be isolated on site if the specifier queries it.
An honest note on scope: we apply electroplated zinc with a trivalent (Cr3+) passivate. Hot-dip galvanizing is a different process with much thicker coatings and different thread allowances — if the project specification requires HDG, it has to be sourced as HDG.

What to send us for an accurate quote

1Drawing or dimensions with thread size, length and the grade or property class (4.6 / 4.8 / 8.8).
2Where the part sits in service: indoors, coastal, buried, cast into concrete, or in a mixed-metal joint. This decides the coating.
3The coating standard and classification if the project specifies one — e.g. an ASTM B633 service condition or an ISO 2081 designation.
4Salt-spray hours and criterion where required, with the significant surfaces defined.
5Thread class, and whether nuts are to be tapped oversize to allow for the coating.
6Lot quantity in pieces or kilograms, piece weight, packing, and the documents required at delivery.

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Recommended services

Zinc Plating Cr3+Heat Treatment Phosphate Coating

Common parts we finish

Anchor Bolts Bolts Nuts Washers All part types →

Questions buyers in this sector ask

Can you hot-dip galvanize anchor bolts?+

No. Our zinc process is electroplating with a trivalent (Cr3+) passivate. Hot-dip galvanizing is a separate process that produces a far thicker coating and needs different thread allowances. Tell us what the project specification actually says and we will tell you honestly whether the part fits our process or has to be sourced as hot-dip.

How many salt-spray hours do I need for outdoor use?+

Salt-spray hours are a comparative quality-control result, not a conversion into years outdoors. Longer results generally follow greater zinc thickness plus a sealer or top-coat: a clear or blue Cr3+ zinc at 5–12 µm typically shows first white corrosion around 72 h and red rust around 120 h, while a sealed system at 8–12 µm typically reaches around 240 h and 480 h. Your specification, not the chamber, sets the number.

Will plated bolts and nuts still gauge after coating?+

Only if the coating allowance was designed in. Zinc grows the effective pitch diameter by roughly four times the local thickness, which is a significant share of the thread tolerance. ISO 4042 covers the allowance, and the usual solution is an oversize tapped nut or a reduced-tolerance thread before plating. Send the thread class and the thickness you need and we confirm the fit before we run the lot.

The bands on this page are typical industry values, not a guarantee for any specific part. The governing drawing, customer specification and standard always take precedence, and every value is confirmed against your part before production. V.S. Heat Treatment operates under ISO 9001:2015; no other certification or sector approval is claimed.