When a bare, uncoated area appears on a galvanized component, the cause is almost never to be found in the zinc bath. Liquid zinc reacts only with pure iron – it will not bind to mill scale or rust. Just a few square centimeters of unremoved oxide is enough to cause under-galvanization. Whether the steel enters the bath properly is determined by whether it is properly prepared. steel etching. We explain what it is, how it works, and why it determines the durability of the entire coating.
What is steel pickling?
Steel pickling is the chemical process of removing layers from the metal surface that develop during production and storage. Each layer interferes with further processing:
- scale (zendra) – a hard layer of iron oxides formed during hot rolling;
- rust - product corrosion, i.e. the reaction of iron with oxygen and moisture;
- oxides and discoloration – thin layers formed, among others, after thermal cutting and welding.
The etching itself involves immersing the elements in acid bath, which dissolves these contaminants and exposes the pure, metallic material. The goal is to bring the surface to a state where subsequent processes—galvanizing, painting, or other coatings—come into direct contact with the metal, not with the layer of contaminants.
Pickling as a surface preparation step before hot-dip galvanizing
In technology hot-dip galvanizing Digestion is not a standalone process, but rather one link in a tightly structured process chain. The standard sequence is as follows:
- Degreasing – removal of oils, greases and organic dirt that acid does not dissolve.
- Digestion – dissolving rust and scale in an acid bath.
- Rinsing – rinsing off any remaining acid and iron salts to prevent them from being transferred to subsequent baths.
- Fluxing – application of a flux (usually zinc ammonium chloride), which protects the clean surface against re-oxidation and improves zinc wettability.
- Galvanizing – immersion in liquid zinc at a temperature of approximately 450°C, where an alloying reaction between iron and zinc takes place.
The quality of the etching determines the zinc's adhesion. The alloying reaction occurs only on the exposed metal – if islands of scale or rust remain on the surface, the zinc will not bond in these areas. The expected level of substrate purity is described in the standards from the series PN-EN ISO 8501.
How does digestion work? Digestion methods and baths.
In industrial practice, pickling is most often carried out by immersion method – structures are immersed in baths of acid solution. The effectiveness and duration of the process are determined by the type of acid, its concentration, and the bath temperature.
Two acids are found in hot-dip galvanizing plants:
| Parameter | Hydrochloric acid (HCl) | Sulfuric acid (H₂SO₄) |
| Operating temperature | Usually the ambient temperature | Elevated (approx. 60–80°C) |
| Digestion speed | High | Requires heating of the bath |
| Surface finish | Smoother, cleaner | Greater risk of over-digestion |
| Universality | Standard in galvanizing | Less common, mainly historical |
Pickling time depends on the condition of the material – slightly rusty steel can be removed in a few to a dozen minutes, while heavily corroded steel requires a much longer immersion time. This is to protect healthy metal from excessive etching and reduce so-called. hydrogen embrittlement, is added to the bath inhibitors – compounds that slow down the reaction of acid with pure iron, but do not hinder the dissolution of rust and scale. Worn-out baths require monitoring of iron concentration and periodic regeneration or replacement.
Pickling stainless steel
At this point, it is worth clearly separating two contexts. Everything above applies to carbon steel (black) prepared for galvanizing. Pickling stainless steel (inox) is a separate process – carried out for a different purpose and with the use of different acids, because stainless steel is not galvanized. This refers to restoration of corrosion resistance and surface aesthetics. Immersion etching of stainless steel, immersing the entire component in a bath, works well for smaller details and allows for even processing of complex shapes.
For pickling stainless steel, a mixture of nitric acid (HNO₃) and hydrofluoric acid (HF) is typically used – in the form of baths, pastes, or gels. The international benchmark for cleaning, pickling, and passivation of stainless steel is the practice of ASTM A380.
Pickling stainless steel after welding
In the weld area, high temperature causes the formation of post-welding discoloration (thermal deposits) and chromium depletion in the surface layer. This is where stainless steel corrodes most easily. Pickling stainless steel after welding removes discoloration and the damaged layer, revealing a homogeneous material with full corrosion resistance.
Etching and the quality of the zinc coating
Returning to galvanizing – this is where the entire logic of the process ends. Zinc coating will only be as good as the surface it hits. A poorly prepared surface means areas not covered with zinc, weaker adhesion, and as a result faster corrosion.
The requirements for ready-made zinc dip coatings are defined in the standard PN-EN ISO 1461 – specifies, among other things, minimum coating thicknesses and criteria for their assessment. Design guidelines and projected coating durability in various environments are collected by PN-EN ISO 14713-1. Meeting these requirements, however, begins long before the galvanizing bath, precisely during the pickling stage. Therefore, in a professional facility like Strumet, surface preparation is treated as seriously as the galvanizing itself.
FAQ – Frequently asked questions about steel pickling
What is the difference between pickling and passivation?
Pickling removes scale, rust, and depleted metal layers from the surface, while passivation rebuilds the protective oxide layer (in stainless steel, chromium oxides). Pickling cleans, passivation protects—often applied one after the other.
What acids are used to pickle steel?
Carbon steel is most often treated with hydrochloric acid, less frequently with sulfuric acid. Stainless steel is pickled with a mixture of nitric and hydrofluoric acid.
How does pickling of stainless steel work after welding?
The component or the weld zone itself is exposed to an acidic bath, paste, or gel, which dissolves thermal discoloration and the damaged layer. After pickling, the surface is thoroughly rinsed and usually additionally passivated.
What happens to steel if it is poorly pickled before galvanizing?
In areas with scale or rust residue, the zinc will not bond, leaving areas of under-galvanization and poor adhesion. This causes the coating to become non-uniform and deteriorate more quickly, reducing the durability of the entire structure.
