Sheet Metal Fabrication

The short answer

Why there is no single best finish

Corrosion is a rate, not a property of a coating. The same 85 micron zinc layer will last more than a century on a heated indoor frame and somewhere between ten and twenty years on a jetty, because the atmosphere around it consumes zinc at a different speed. ISO 9223 sorts atmospheres into categories from C1, a heated interior, to CX, an offshore splash zone, and the first-year zinc loss rises from about 0.1 microns to 8.4 microns a year across that range, with uncoated steel losing between 1.3 and 200 microns in the same year. Until you know which category the part will live in, any claim that one finish is "best" is a guess.

So the honest answer has two parts. If the part can stay unpainted and the environment is C3 or worse, hot-dip galvanizing is the benchmark, because it protects by sacrificing zinc rather than by sealing the surface. If the part must be a colour, or the environment is C4 to C5, a duplex system of powder over galvanizing is the usual specification. Everything else on this page is the reasoning behind those two sentences, and the cases where neither applies.

The finishes ranked for outdoor exposure

Six finishes account for most outdoor sheet metal work, and they fail in different ways. Hot-dip galvanizing immerses the fabricated part in molten zinc, forming zinc-iron alloy layers that are metallurgically bonded to the steel; the coating is thick, abrasion resistant and cathodic, so a scratch does not become a rust site. Powder coating over bare steel is a barrier only: it is tough and UV stable while intact, but any pinhole, cut edge or scratch exposes steel that has no protection of its own, and rust creeps under the film from that point. A duplex system combines the two, with the powder giving colour and UV resistance and the zinc giving sacrificial protection at every damaged point. E-coat is a thin, uniform epoxy film that reaches recesses and edges well but degrades in sunlight unless it is topcoated. Zinc plating is electrodeposited and therefore thin, typically 5 to 15 microns, which is adequate indoors and inadequate at the coast. Anodizing works only on aluminium, where the oxide layer is grown from the metal itself and cannot chip off, but it is brittle and colour fades in strong UV. Bare 316L stainless relies on its molybdenum content for pitting resistance and needs no coating at all in marine air, at a material cost several times that of coated carbon steel.

The one row that surprises buyers is powder on bare steel. A good polyester powder at 70 to 80 microns will pass 750 hours of salt spray on a test panel, which reads like a decade of protection, and on a real fabrication it often fails at the cut edges first, because those edges were never coated to the same thickness. This is why the same powder performs far better over galvanizing: the edge is protected by zinc even where the film is thin.

Zinc thickness sets the service life

Once the substrate is galvanized, life becomes arithmetic: coating thickness divided by the local zinc loss rate. The rate comes from the corrosivity category, and the thickness comes from ISO 1461, which sets minimum mean coating masses by article thickness rather than one number for everything. That detail matters for sheet metal specifically, because sheet parts are thin. A 6 mm plate is guaranteed 85 microns, a 3 mm sheet 70 microns, a 1.5 mm sheet 55 microns and anything under 1.5 mm only 45 microns. The same galvanizing bath therefore produces a coating on a 1.2 mm enclosure panel that will last roughly half as long as the coating on a 10 mm base plate, and no amount of specification writing changes that.

Two cautions on that table. The rates are first-year figures, and zinc corrosion slows as the surface patina builds, so real life is usually longer than a straight division suggests. And the table assumes sound galvanizing at the full coating thickness; a 45 micron coating on thin sheet lands at roughly half the life shown, which is why thin-gauge outdoor parts are often specified as duplex rather than galvanized alone.

Where each finish fails outdoors

Powder on bare steel fails at edges, holes and scratches, then creeps: a rust blister lifts the film and holds water against the steel. Hot-dip galvanizing fails at welds and seams that were not sealed, in trapped water, and through white rust if parts are stacked wet in storage before installation. It also distorts thin panels in the molten bath and fills threaded holes and close-tolerance bores unless they are masked or re-tapped. Anodizing fails where the oxide is cracked by bending after coating, and its colour fades under UV even though the corrosion protection remains. Zinc plating fails simply because it is too thin for chloride exposure; 168 hours of salt spray is a few months at the coast. E-coat fails in sunlight unless a topcoat is added, and its thin film offers little abrasion resistance on handling edges. Bare 316L fails only in warm chloride environments where pitting can start, typically above 60 degrees Celsius or in stagnant seawater.

Design rules that keep a coating intact

Coatings fail at geometry long before they fail at chemistry. Any closed hollow section that will be galvanized needs vent and drain holes, sized and placed so the zinc can flow in and out and no air pocket is trapped; the same holes prevent a blowout in the bath. Lap joints, hemmed edges and overlapping panels should be avoided or sealed, because they create a crevice that holds electrolyte and cannot be coated. Sharp external edges should be broken with a small radius, since a paint or powder film pulls thin over a sharp corner and that is where corrosion starts. Mixed fasteners are a similar problem: a stainless bolt in a galvanized bracket creates a galvanic couple that eats the zinc locally. Finally, do not mix finishes within one assembly unless you intend the difference, because a painted bracket bolted to a bare galvanized frame will show a colour mismatch and will also concentrate coating damage at the joint.

What to put on the drawing

A corrosion specification needs five numbers or phrases, and most drawings carry only one. State the substrate grade, the finish process and its standard, the coating thickness in microns with a minimum as well as a nominal, the test method and hours with pass and fail criteria, and the areas to be masked. "Powder coat black" is not a specification; "polyester powder, 70 to 90 microns, over hot-dip galvanized to ISO 1461, 1,000 hours neutral salt spray to ASTM B117 with no red rust and no blistering above rating 8, threads masked" is. The test method matters because ASTM B117 and ISO 9227 define the apparatus but not the acceptance criteria; the buyer has to write those, and a supplier cannot be held to a limit that was never stated.

See surface finishing for the coating processes we run on fabricated parts, sheet metal fabrication for cutting, forming and assembly before coating, and custom sheet metal parts for the bracket and enclosure work these finishes are usually applied to.

The short answer

Why a bend with no relief tears

When sheet metal is formed, the outside of the bend stretches and the inside compresses, so material has to move along the bend line to accommodate the change. Where that line ends at the edge of the sheet, at a notch, or at another flange that is already formed, the material has no direction left to flow in. The strain then concentrates at the very end of the bend line: the outer fibre exceeds what the sheet can take and the corner tears, or the material piles up and bulges outwards instead. The failure is created in the flat pattern, not at the press brake, which is why a design that looks perfect in a 3D model can arrive as a box with exploded corners, sharp burrs and a deformed adjacent flange. Cutting a small piece of material away before forming gives the bend somewhere to go; that cut is the relief.

The two rules that size a relief

Two dimensions matter, and both are tied to the material thickness and the inside radius rather than to a fixed number. The relief must be at least one material thickness wide, with about twice the thickness a comfortable working value. It must be at least as deep as the bend radius plus one thickness, and roughly the bend radius plus 1.2 times the thickness if you want margin. The depth rule has a physical reason: the relief has to reach past the bend line, because a cut that stops short of it leaves the flange to tear exactly as it would with no relief at all. A worked example is easier than the algebra. For a 2 mm aluminium part with a 2 mm inside radius, the minimum relief is 2 mm wide and 4 mm deep, and 4 mm wide is the value that gives the tooling room and leaves a visibly deliberate notch rather than a ragged corner.

Three relief shapes and when to use each

A rectangular notch is the standard choice for laser-cut and punched work: it removes material cleanly, produces no burr, is trivial to program, and the only cost is a visible gap in the corner of the finished part. A tear relief removes nothing at all, simply slitting the corner so the metal tears along the slit as it bends; it gives the tightest corner with no visible gap, which suits a punched part where the corner will be hidden or welded, but the torn edge is jagged and sharp and can cut fingers or wire harnesses. An obround or rounded slot is the choice for parts that will see vibration or repeated load, because the rounded end spreads the stress concentration instead of leaving a sharp inside corner for a crack to start from; it is slightly slower to cut in thick material and needs the flat pattern checked. Whichever you choose, generate reliefs with the sheet-metal tool in the CAD system rather than drawing them by hand, so the geometry stays consistent with the thickness and radius actually in use.

Relief against the other rules near a bend

A bend relief is not a substitute for the rest of the sheet-metal spacing rules, and it is often confused with them. Holes and slots must sit clear of the bend line, generally at least 2.5 times the material thickness plus the bend radius away, with about 3 times the thickness as a safer figure for critical features; too close and the hole distorts into an oval as the material flows. Flanges have to be long enough for the press brake to grip them: about four thicknesses plus the bend radius is the minimum that forms consistently, and anything shorter needs special tooling or a secondary operation. The inside bend radius itself should be at least one thickness for mild steel and one and a half to two thicknesses for stainless and hard tempers, or the outer fibre cracks regardless of how good the relief is. In other words, a relief protects the end of a bend line; it does nothing for a hole placed too near the middle of one.

What a missing relief costs downstream

The immediate symptom is a torn corner and a burr that has to be dressed by hand, which is a cost in labour and a visible quality defect on any enclosure that a customer will handle. The second symptom is quieter and worse: material bulging outwards at the bend radius means the flange face is no longer flat. A lid or a mating panel then rocks on the corner bulges instead of seating, fasteners pull the joint down and distort the panel, and what looked like an assembly problem turns out to be a forming problem. The third symptom appears later, when a sharp unrelieved inside corner acts as a stress raiser and a crack begins under vibration or thermal cycling; a rounded relief changes that corner into a smooth transition and removes the initiation site. All three are avoided by a cut that takes less than a second on a laser.

How to specify it on the drawing

Show the relief on the flat pattern rather than describing it in a note, because that is the drawing the laser or punch actually runs from, and give its width and depth as dimensions rather than leaving them to the shop. State the material thickness and the inside bend radius for every bend, since both rules above are derived from them and neither can be assumed from the sheet gauge. Note the bend direction and, where crack risk is high, ask for bends across the grain rather than along it. Keep the bend radius consistent across the part so a single tool set can form it, and flag any flange that falls below the four-thickness minimum early, because that is a tooling decision rather than a drawing one. Finally, mention any surface finish that follows, since a powder coat or paint film will be thinner or missing on the inside of a narrow notch and the gap will be more visible after coating than before it.

See sheet metal fabrication for the laser cutting and forming route we run, metal bending for press brake work quoted from your flat pattern, and metal stamping for tooled parts where a tear relief or a punched notch is built into the die.