Mine Coated Bolts & Square Weld Nuts
M8 x 45, M10 x 45 and M12 x 45 bolt and square weld nut sets, finished in Mine Coat: a protective coating built to resist rust for longer than standard zinc-plated hardware in the damp, corrosive conditions found underground.
Independently salt-spray tested, not just rated on paper. Supplied as matched sets: a bolt paired with a square weld nut (not a hex nut), standard metric thread.
Frequently asked questions below ↓
Why Mine Coated Fasteners
Lasts Longer Underground
Mine Coat is a protective coating built to resist rust for longer than standard zinc plating, in the damp, chemically active conditions typical of underground mining.
Built On a Recognised Standard
The base zinc coating follows ISO 2081, the international standard fastener manufacturers use for zinc-plated hardware, so the starting point is a well-proven coating, not a one-off process.
Independently Tested, Not Just Rated
Lab-tested in a salt-spray chamber that speeds up corrosion, giving a real measured result (480+ hours to first rust spotting) rather than a generic manufacturer claim.
Standard Metric Sizing
M8 x 45, M10 x 45 and M12 x 45, standard coarse thread. The bolts fit standard washers of the corresponding size; the nuts supplied are square weld nuts, not hex nuts (see below).
Even Coverage on the Thread
The coating is applied evenly over the whole thread, not just the surface, so bolts turn and tighten the way you'd expect without the coating building up unevenly.
Matched Bolt & Square Weld Nut Sets
Each bolt is supplied with a square weld nut, not a hex nut, so the pair is ready to use for the plate-welded fixing points common in mine steelwork.
Structural Fastening Use
Suited to structural steel connections, equipment mounting and general fastening in corrosive underground conditions.
Stocked Locally
Held in stock in Johannesburg for fast turnaround on standard orders.
The Coating
What Is Mine Coat?
In plain terms: Mine Coat is a rust-resistant finish, similar in idea to ordinary galvanised or zinc-plated hardware, but built in three layers so it holds up roughly twice as long before rust sets in, in the damp and chemically active conditions found underground.
Mine Coat is STOTKO's name for that three-layer finish, built specifically for fasteners used in underground mining conditions, where humidity, water ingress and chemical exposure wear through plain zinc plating faster than in a typical above-ground setting.
The first layer is a base coat of electroplated zinc, applied to the international standard fastener makers use for zinc plating (ISO 2081). This is the main protective layer, and it works the same way ordinary zinc plating does: it corrodes first, protecting the steel underneath.
The second and third layers are a yellow passivate coating plus a clear sealer on top. Think of the passivate as converting the zinc surface into a tougher, more rust-resistant layer, and the sealer as an extra waterproof topcoat over that. Together, the three layers give meaningfully longer rust protection than a bare zinc-plated fastener at the same thickness.
The base fastener itself is manufactured from mild steel to ISO 898-1 property class 4.8 (stamped on the bolt head), giving a defined minimum tensile and yield strength before the Mine Coat finish is applied. Unlike many coated fasteners on the market, the electroplating is done in-house at STOTKO rather than outsourced, so coating thickness and quality are controlled end-to-end on every batch.
Test Data
Salt-Spray Performance
A salt-spray test puts a coated part in a chamber that sprays a continuous salt fog, speeding up corrosion so a coating's real-world lifespan can be measured in days instead of years. The figures below are from that test (standard method ISO 9227), at a coating thickness of 12 micron.
In practical terms: the longer a fastener lasts in this test before rust appears, the longer it should hold up in real underground conditions before it needs replacing.
12 Micron Coating
The coating thickness tested for the figures on this page. Thicker coatings generally last longer.
480+ Hours Before First White Spotting
White corrosion is a light surface film that appears first, well before any structural rust. 480 hours in the test chamber is roughly 20 days of continuous salt exposure.
600+ Hours Before Red Rust
Red rust is the point where the underlying metal itself starts to corrode. 600+ hours is roughly 25 days of continuous salt exposure before that happens.
Class 3 (C3) Corrosivity Rating
A standard industry rating for "medium" corrosion exposure, the same category used for humid indoor spaces and sheltered underground areas.
ISO 2081 Base Coating
The underlying zinc plating meets ISO 2081, the standard international benchmark for zinc-plated fasteners.
Contains Hexavalent Chromium
The yellow passivate layer contains Cr6+, a chemical standard for this type of yellow coating and used across the fastener industry. Handle and dispose of coated parts per your site's normal Cr6 material procedures.
Technical Reference
Available Sizes
Supplied as matched bolt and square weld nut sets in standard metric coarse thread. Note: the nut supplied is a square weld nut, not a hex nut.
| Size | M8 x 45 | M10 x 45 | M12 x 45 |
|---|---|---|---|
| Thread | M8, coarse | M10, coarse | M12, coarse |
| Length | 45 mm | 45 mm | 45 mm |
| Nut Type | Square weld nut | Square weld nut | Square weld nut |
| Coating | Mine Coat | Mine Coat | Mine Coat |
| Coating Thickness | 12 micron | 12 micron | 12 micron |
| Corrosivity Class | C3 | C3 | C3 |
| Supplied As | Bolt + square weld nut | Bolt + square weld nut | Bolt + square weld nut |
Coating thickness and salt-spray figures are as tested per the coating supplier's data. Contact us for the full test report. Need hex nuts instead of square weld nuts? Let us know when ordering.
Applications
Where Mine Coated Fasteners Are Used
Structural Steel Connections
Bolted connections on structural steelwork exposed to underground humidity and moisture.
Equipment Mounting
Fastening for pumps, conveyors and other fixed mining equipment.
Shaft and Tunnel Infrastructure
General fastening on shaft steelwork and tunnel support structures.
Wet or Chemically Active Areas
Areas with water ingress or chemical exposure where plain zinc plating wears through quickly.
General Maintenance Fastening
Replacement fasteners for maintenance work where corrosion resistance matters more than plain zinc plating offers.
Industrial Sites Outside Mining
Any C3-classified corrosive environment, not limited to underground mining.
Why STOTKO
Why Buy Mine Coated Fasteners From Us
Tested, Not Just Rated
Salt-spray figures on this page come from actual testing of the coating, not a generic manufacturer claim.
Built for Underground Conditions
The coating system was chosen specifically for the humidity and corrosion exposure found in mine shafts and tunnels.
Held In Stock
Standard sizes stocked in Johannesburg, no waiting on production runs for standard orders.
50+ Years of Engineering
STOTKO Engineering has supplied mining equipment in South Africa since 1973.
FAQ
Frequently Asked Questions
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