A properly formed hot dip coating is alloyed into the steel and does not come off. When it does, the cause is almost always identifiable — and usually decided before the piece reached the kettle.
Hot dip galvanizing is not a layer sitting on the steel. The zinc reacts with the iron to grow a series of zinc-iron alloy layers that are metallurgically part of the section, with a layer of free zinc on the outside. That structure is why a correctly galvanized member survives decades of handling.
So peeling is not normal wear — it is a signal. In practice it comes from one of three places: the steel was not chemically clean when it went in, the coating grew too thick and brittle because of the steel's own chemistry, or the piece was mechanically damaged afterwards.
| Cause | What you see | Where it was decided |
|---|---|---|
| Surface contamination | Bare or thinly coated patches with sharp edges, often at welds or in a pattern matching a mark or a handling point. Coating lifts cleanly, leaving grey steel. | Before the kettle — paint, lacquer, marking crayon, weld slag or anti-spatter that survived the degrease and resisted the pickle. |
| Excessive coating on reactive steel | A thick, dull grey, matte coating that flakes off in plates, usually across a whole member rather than in patches. Often thicker than the specification requires. | In the steel's chemistry — silicon content in the reactive range grows the alloy layers faster than the process can control. |
| Mechanical damage | Bright scratches, chipping at edges, or coating removed where a chain, sling or forklift touched. | After the kettle — handling, transport, or site erection. |
The one that surprises people is the middle row, because nothing was done wrong at the galvanizing plant. The steel decided the outcome.
The rate at which zinc-iron alloy layers grow depends heavily on the silicon content of the steel. In certain ranges the layers grow much faster and keep growing, producing a coating that is thicker than intended, dull grey rather than bright, and brittle enough to flake.
ASTM A385 recommends silicon either below 0.04% or between 0.15% and 0.22%. The problem zone sits between those figures — the reactivity curve is steep around 0.05%, which is why two heats that look almost identical on a mill certificate can behave completely differently in the kettle.
| Silicon content | Typical coating behaviour |
|---|---|
| Below 0.04% | Bright, shiny coating at the thickness the standard expects |
| About 0.04% to 0.15% | The reactive zone — thick, dull grey coating; brittle and prone to flaking |
| 0.15% to 0.22% | Returns to controlled, acceptable coating growth |
A mill certificate gives one silicon figure for a heat, not a distribution across every section rolled from it. That is why reactive behaviour can appear on part of a consignment and not the rest — and why it is worth telling your galvanizer if you are supplying steel of unknown provenance.
Galvanizing is on our own floor at Howrah, which means fabrication cleanliness and coating are under one quality system rather than two. We run a Bruker Q2 ION optical emission spectrometer on site — imported from Germany, commissioned June 2026 — so steel chemistry can be checked rather than assumed, and coating thickness is read on a calibrated DFT gauge and reported per batch.
Where a coating comes out reactive-grey, we say so and explain why. It is a real and well-documented metallurgical effect, and pretending otherwise helps nobody.
Answered from our own line, not from the standard's cover page.
Almost always steel chemistry rather than process. Silicon in the reactive range grows the zinc-iron alloy layers all the way to the surface, so there is no free zinc left to give the bright finish. The coating is usually thicker than required and duller, and it can be more brittle. It is not a defect in the galvanizing.
Not in corrosion protection — it is usually thicker, and the alloy layers are what actually protect the steel. It is less attractive and it can be more brittle, which matters if the piece will be handled hard or bent afterwards.
ASTM A385 recommends below 0.04% or between 0.15% and 0.22%. The zone between those figures is where reactive behaviour appears, and the curve is steep around 0.05%.
Small damaged areas are repaired to ASTM A780 using zinc-rich paint, zinc solder or thermal spray, depending on the area and the specification. A member that is flaking across its whole length is a different problem — that usually needs stripping and re-dipping.
Yes, in house, on a Bruker Q2 ION optical emission spectrometer reading steel, zinc, aluminium and copper. If you send steel for job work with a mill certificate, send that too.
Tell us the standard, the coating requirement and the component, and we will confirm in writing what we can hold and how we will certify it. Galvanizing is on our own floor, so the answer comes from us and not from a sub-contractor.
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