Hot-rolled mild steel corrosion and finish compatibility

Hot-rolled mild steel corrosion is decided by the surface the mill supplied, not by the coating added later. The grey-blue mill scale on an as-rolled 3 mm plate is a dense iron oxide, and it sits cathodic to the steel beneath it. Where that scale breaks — a laser cut, a sheared edge, a scuff from a fork tine — the exposed steel becomes a small anode coupled to a large cathode, and it pits instead of rusting evenly.

At exposure category C4, ISO 9223 places carbon steel at 50–80 µm of section loss per exposed face per year. A 3 mm plate exposed on both faces loses 1.0–1.6 mm over ten years, which is a third to a half of its thickness. No 70 µm powder film changes that arithmetic. Only the surface preparation and the finish system change the rate, and the finish has to match the exposure the part actually sees rather than the one on the enquiry form.

This page is for the design engineer who has to pick between bare, painted, powder coated and hot-dip galvanised hot-rolled steel, and defend the choice to a quality team three years later. It gives the exposure table we quote against, the preparation each finish needs, the numbers for a 3 mm plate, and the drawing notes that stop a mill-scale part reaching the paint line.

What drives hot-rolled mild steel corrosion in service?

Two variables do the work: the surface condition at the start, and the corrosivity category of the place the part is installed. Everything else — colour, gloss, brand of powder — is downstream of those two.

The surface decides whether the loss is uniform or localised. Blasted steel under a coating corrodes from a coating defect outward and stays broad and shallow. As-rolled steel with intact scale concentrates its galvanic current at every break in that scale, and the ratio is against you: one 1 mm² pinhole in a 10,000 mm² plate has to carry a current density ten thousand times what the same current would put on the full face. That is why a mill-scale bracket can pass a film-thickness gauge and show a perforation in a rail channel two winters later.

The category sets the rate. ISO 9223 classifies atmospheres from C1 (heated, dry interior) to CX (offshore, tropical, aggressive), and its first-year carbon steel rates are the numbers that get written into a corrosion allowance. The same table gives zinc and aluminium rates, which is what makes a coating comparison possible in years rather than adjectives.

Read the table as a filter, not a menu. A part at C4 with a 3 mm section has 1.0–1.6 mm of steel to give over ten years, which is 33–53% of the plate, and a bolted joint that depends on clamp load will loosen long before the plate perforates. A part at C5 has to move to a metallic coating or a different alloy, because an organic film at 120 µm cannot hold a 200 µm/yr front.

Why does mill scale have to come off before powder or paint?

Because no production pretreatment tank removes it at line speed, and no powder bonds to it. ISO 8501-1 defines the blast grades the drawing should name: Sa 1 light blast, Sa 2 thorough, Sa 2½ near-white, Sa 3 white metal. As-rolled scale with a solvent wipe is not a grade in that standard at all, and the failure it produces is misdiagnosed as a coating fault.

The mechanism is mechanical as much as chemical. Scale is brittle, it sits 10–50 µm thick on a hot-rolled surface, and it is already micro-cracked from coiling and uncoiling. The heat of a curing oven and the differential expansion between oxide and steel finish the job: the film has bonded to a layer that is now separating from the substrate, and it lifts in patches. Under a powder coat the patch presents as a blister with a rust nucleus, and it appears in the first wet season, not on the first inspection.

• Blast to Sa 2½ minimum, with angular grit, and check the profile. 40–70 µm Rz holds powder; above 100 µm the peaks stand proud and pinhole; below 30 µm the mechanical key is gone.
• Coat inside four hours of blasting. Flash rust forms on a blasted surface within hours in a coastal shop, and a phosphate cannot convert what is already oxidised.
• Blast before the seven-tank line, not instead of it. The tanks degrease and phosphate; they do not remove mill scale at 3 mm.
• Do not pickle hot-rolled steel to save blasting on thick plate. Acid pickling removes scale but leaves smut and hydrogen, and the treatment time for 12 mm plate is not a line operation.

Pickled and oiled hot-rolled sheet, usually called HRPO, is the middle route. The mill pickles the coil, which removes scale and leaves a surface that takes phosphate directly, so the shop can skip the blast chamber on 1.5–4 mm parts. It costs more per tonne than as-rolled, and it rusts freely once the oil is gone, so it suits a plant that can move the part to pretreatment within days. For a 6 mm and above plate, blasting is cheaper than buying pickled.

Which finish system holds in which exposure?

Powder, wet paint, hot-dip zinc and bare each own a band of the exposure table. Choosing across bands is where most of the cost arguments start, so it is worth putting numbers on each.

Powder over blasted steel

A 60–80 µm polyester or epoxy-polyester powder over a zinc or iron phosphate is the standard industrial answer at C2 and C3. ISO 12944-5 bands paint durability as low (2–5 years), medium (5–15) and high (15–25), and a single-coat powder at 80 µm sits in the medium band at C3. At C4, the same system drops to the bottom of medium or below, because the film is intact at 80 µm but the steel at every cut edge is not.

Film build has a ceiling as well as a floor. Above about 120 µm, powder loses impact resistance, cracks at folded edges and chips at bolt holes. The failure looks like a coating problem and is a film-build problem.

Wet paint

Two-coat wet paint runs 100–140 µm total with an epoxy primer and a polyurethane topcoat, and it is the right answer where the part is large, brushed or touched up on site, or where the colour has to match an existing machine. It gives the same order of protection as powder at similar film build, with the advantage that a scratch can be repaired in the field and the disadvantage that solvent emission and cure time cost more per square metre.

Hot-dip galvanising

ISO 1461 sets the minimum average coating thickness by article thickness: 45 µm for 1.5–3 mm, 55 µm for 3–6 mm, and 70 µm at 6 mm and above, with local minima 10–15 µm below those figures. Practice runs higher — a 6 mm bracket often comes back at 85–120 µm — and the zinc rate at C4 of 2.1–4.2 µm/yr turns 85 µm into 20–40 years.

The cost is thermal. The bath runs at 445–460 °C, which warps thin panels, relieves the stress in cold-formed sections and changes the flatness of anything under about 3 mm. Threads need oversize tapping or chasing after galvanising, because 55 µm per surface closes an M8 coarse thread by 110 µm across the flanks. Closed sections need vent and drain holes of 8–10 mm, placed where the zinc can run out.

Duplex: zinc plus paint

Zinc under paint is the longest-life system on hot-rolled steel in C4 and C5, and it works because the two protections fail differently. The paint slows the zinc consumption; the zinc protects the steel at every scratch the paint takes. Published duplex factors run 1.2–2.5 on the sum of the two individual lives, so a 20-year zinc life plus a 10-year paint life lands between 36 and 75 years, and the conservative end of that is the number to quote.

Bare and oiled

Bare hot-rolled steel is correct in C1 and C2 interiors, for structural parts inside a machine guard, and for parts that are machined after forming. It needs a corrosion allowance, typically 0.5 mm at C2 for a ten-year life, and it needs the oil film maintained. Bare steel in a wash-down area is a maintenance item, not a design.

How do edges, welds and threads behave under each finish?

Edges are where every finish is weakest, and they are the part of the drawing that gets the least attention.

A laser-cut edge on 6 mm hot-rolled steel carries an oxide layer that the phosphate does not convert, and a cut face that powder can bridge but not seal at full thickness. The oxide band runs 0.1–0.3 mm into the edge on laser-cut sheet. Deburring to a 0.2 mm radius before coating lets the film wrap the corner, and that single change moves the first rust point from the edge to the face in most C3 installations.

Welds lose their coating by definition, and zinc-coated stock loses more than that. A MIG weld on galvanised sheet burns the zinc back 5–10 mm each side of the bead, leaving a bare strip that rusts first, and the weld itself is porous unless the zinc is ground off before welding. On hot-rolled steel the equivalent problem is scale: welding through mill scale gives porosity and slag inclusions that a leak test finds later.

Threads and fits shrink by the coating. A powder film at 70 µm closes a 0.20 mm clearance joint completely once both faces are coated, and hot-dip zinc at 55 µm per surface closes an M10 thread. Specify masked threads, oversize tapping, or weld nuts fitted before coating, and say which on the drawing.

How is hot-rolled steel finished at Ganesh?

Heavy-gauge hot-rolled work enters through the 12 kW fibre laser on a 40 ft × 16 ft bed, which cuts mild steel to 20 mm, and the iron worker handles plate to 30–40 mm where the profile is simple. Forming runs on a 200 T, 5-axis press brake with a 4,450 mm bed to 10 mm thickness. Above that, the part is cut and welded rather than bent.

Surface preparation is the part that decides the outcome. The 16 ft × 16 ft shot blast chamber takes fabricated frames and plate that the tanks cannot descale, and the seven-tank pretreatment line follows with alkaline degrease, rinse, conversion coating, rinse and dry-off. Powder goes on in an 18 ft × 14 ft electrostatic booth, with a wet paint booth and curing oven for two-coat work. Dry film thickness is measured on every batch and cross-hatch adhesion is tested to the drawing's class.

The company has fabricated since 1992, runs ISO 9001:2015 and ISO 14001:2015, and operates five units in Mumbai (Vasai) and Nagpur. Parts here run from 0.8 mm to 40 mm, formed tolerance is ±0.5 mm typical with tighter features by agreement, and PPAP or first article inspection is available where the programme needs it. Customers include Mahindra, Mahindra Powerol, Godrej & Boyce, TKE, Eros Elevators, Minda, Gunnebo and Indian Railways.

When hot-rolled steel with a coating is the wrong route

Hot-rolled steel is the wrong substrate below about 2.5 mm. At 2 mm the mill thickness tolerance is roughly ±0.16 mm against ±0.08 mm for cold rolled, the surface carries scale and waviness that shows through a gloss finish, and the price advantage per kilogram does not cover the extra blasting and inspection. Cold rolled sheet, phosphated and powder coated, is the better part.

It is also wrong where the coating has to survive immersion, splash or chemical contact. A 3 mm hot-rolled tank panel at C5 will fail at the weld toes whatever the film build, and 316L is the answer rather than a thicker coating.

We do not hot-dip galvanise in-house, we do not anodise or electroplate aluminium, and we do not run a coil coating line. Where the drawing needs a continuously pre-painted or metallic-coated coil, the substrate is bought coated and the forming limits change with it. Where the part needs zinc, it is fabricated here and galvanised outside, which adds a transport leg and a distortion risk that should be priced into the decision, not discovered at assembly.

What to put on the drawing

• The material standard and grade, for example IS 2062 E250A or EN 10025-2 S235JR, with the delivery condition (as-rolled, pickled and oiled).
• Thickness with its tolerance class, and the faces that matter for flatness.
• The exposure category, C2 to CX, or the installation description if the category is not known.
• Surface preparation grade to ISO 8501-1, and the profile range in µm Rz.
• The finish system by layers: pretreatment type, primer, topcoat, colour and dry film thickness range in µm.
• Edge condition: deburred radius, and whether edges need sealing before assembly.
• Weld locations, whether the weld is coated after assembly, and any leak test.
• Threaded features and fits, with the masking or oversize instruction.
• The adhesion test class and, where specified, the neutral salt spray duration to ISO 9227 with separate white-rust and red-rust limits.

Send the drawing with the exposure and the annual quantity to request a quote, and the route is confirmed before the first blank is cut. The materials and grades hub carries the grade-level guidance that sits behind this finish decision, the degreasing guide for mild steel covers the first tank in detail, and the phosphating guide covers the conversion layer the powder bonds to. Blasting capacity and chamber size are on the shot blasting capability page.