Zinc Coating Defects: Runs, Grey Coating, Detachment and Black Spots

How to use this reference

This article collects the defects caused by surface preparation, bath conditions and handling after dipping: runs, black spots, blisters, grey coating, flaking and others. Defects of the base material and the structure, and corrosion during storage, are covered in Hot-dip Galvanising Defects: Causes and Prevention, which also explains how a good coating forms.

For each defect the signs, causes and ways to prevent it are given. Acceptable thickness and the allowances for repairing uncoated areas are in Zinc Coating Thickness under DSTU EN ISO 1461:2024; how to prepare a structure for the bath so as to avoid most of these defects is in Designing Steel Structures for Hot-dip Galvanising and in the preparation checklist.

Defects linked to surface preparation and the bath

Flat runs

Signs: thickened zinc areas in the form of stripes standing above the coating; drops may collect at the base of the article’s contour.

Causes: the speed of withdrawal does not match the viscosity of the molten zinc; fine iron-zinc crystals formed from iron salts and scale; withdrawing too fast, especially with aluminium; breaking of the aluminium oxide film, for example by impact.

How to avoid: rinse off iron salts thoroughly after pickling, regenerate flux with a high iron content, and do not shake articles during withdrawal.

Drip runs (“tears”)

Signs: drops of zinc that have solidified on the article after withdrawal.

Causes: high surface tension of the melt where zinc oxide forms; the oxide arises from scale left on the article.

How to avoid: remove scale from articles; zinc oxide in the bath is removed by deoxidising.

Black spots

Signs: irregularly shaped areas with no zinc coating.

Causes: contamination on the surface (moulding sand, welding residues, slag, grease, paint); articles touching each other during immersion; flux burnt out through overheating in drying.

How to avoid: remove sand and slag by shot blasting, paint and grease by degreasing, prevent parts from sticking together, replace spent flux and do not overheat it when drying.

Pitting attack

Signs: large, deep destruction of the article in steels with a high sulphide, oxide and slag content.

Causes: over-pickling, in which the acid attacks sulphides and oxides.

How to avoid: shorten pickling time.

Blisters from pickling

Signs: small bumps on the galvanised surface, where the coating has lifted from the steel.

Causes: lamination of the material under the action of hydrogen, typical of low-strength steels; on heating in zinc the hydrogen is released as bubbles.

How to avoid: quick, strong pickling.

Hydrogen embrittlement, delayed fracture

Signs: the part fails not immediately but under load; a zone of initial cracking is visible on the fracture; the cracks are hard to tell apart from others.

Causes: hydrogen penetrates the steel during pickling and accumulates at grain boundaries; typical of high-strength bolts and heat-affected zones of welds.

How to avoid: high-strength steels are better pickled in hydrochloric acid of high concentration (in sulphuric acid, by contrast, hydrogen uptake grows with concentration).

Pickling and flux residues

Signs: red-brown spots and runs on the galvanised surface at cracks, cavities and pores.

Causes: residues of pickling and flux solutions in pores and cavities come out onto the surface; a large amount can even corrode the steel from inside.

How to avoid: the design must let residues escape during galvanising; the article must have no pores.

Lumps

Signs: small bumps of various shapes, singly or in groups.

Causes: impurities in the bath (hard zinc, aluminium-iron compounds) settle on the article on withdrawal; hard zinc precipitates when the top of the bath is cooler than the lower part.

How to avoid: remove the particles from the bath, keep the temperature even, remove aluminium-iron compounds from the surface.

Lead foam

Signs: uneven bumps of various shapes and sizes.

Causes: lead separates from the zinc and sinks to the bottom, oxidises, and after melting ends the lead saturated with zinc oxide settles on the articles.

How to avoid: stir the contents at the bottom of the bath so the particles float up, and remove them.

Discolouration of the coating

Signs: areas or stripes with a gloss and colour different from the white zinc oxide film; they can appear immediately or after exposure to air; visible at an angle.

Causes: traces of foreign elements in the zinc (copper, manganese, antimony) form coloured oxides.

How to avoid: add high-purity zinc to the bath.

Drops and drips

Signs: drops or thin deposits of zinc; they can be inside pipes, especially on the welded grating.

How to remove: drips are removed by vibration, drops mechanically with files or brushes.

Ash and flux residues

Signs: grey or yellowish non-metallic deposits of zinc oxide and zinc chloride; they stick firmly and clog corners.

How to avoid: clean the ash carefully from the bath surface before and after withdrawal; ash in the corners is removed with a brush (areas with slight corrosion may remain).

Sticking (contact marks)

Signs: places where parts galvanised together stuck and the coating tore when they were separated.

How to avoid: when galvanising several parts at once, make sure they do not touch each other as they are withdrawn from the bath.

Imprints

Signs: dents in the zinc surface made by a mechanical action; the shape depends on the tool or object.

How to avoid: do not touch the zinc coating with tools or objects before it has set.

Grey coating

Signs: a dull, uniform or light grey coating (all over or only in places); it consists only of iron-zinc alloy with no continuous outer zinc layer.

Causes: steel with a critical silicon content (steels above 0.2% Si give a thick grey coating), thick-walled articles, and also too slow cooling or reheating.

How to avoid: hard to prevent in thick-walled articles and steels with critical silicon; fast cooling after withdrawal can prevent grey coating.

Coating detachment

Signs: damage where the zinc coating together with the iron-zinc alloy separates from the base; it occurs on a thick alloy layer, on a very smooth surface or along the edge of the article.

Causes: impacts, elastic or plastic deformation of the part creating shear stress.

How to repair: the area can be covered with zinc-containing paint compositions.

Flaking (detachment of the top zinc layer)

Signs: the outer zinc layer separates from the iron-zinc alloy under it.

Causes: the reaction between zinc and steel continues below the melting point of zinc; pores form at the boundary of the layers (the Kirkendall effect). It happens when hot galvanised articles are stacked and cool slowly, or when a structure works for long at about 200 °C; a small lead content in the zinc contributes.

How to avoid: cool the article below 200 °C after galvanising; do not use galvanised articles at high temperature.