Inside the Dip: Why Bare Spots, Thick Coats, and Peeling Still Plague Hot‑Dip Galvanizing

In galvanizing, the smallest misses can balloon into costly rework. Industry data note that repair of severe bare areas can increase project costs by 10–30%, while standards such as ISO 1461 allow only very limited bare spots (ckit.co.za). The root causes—usually in cleaning, pickling, and fluxing—are well known and measurable, as are the process setpoints in the zinc bath.

The American Galvanizers Association and other guides make the same point in plain terms: steel will not react with zinc to form a coating unless the surface is completely clean (agindo.org). ASTM A123 (widely referenced in Indonesian SNI 07‑7033) and ISO 1461 then set the acceptance bands for coating thickness and surface condition (bondigalva.co.id).

Bare spots: pre‑treatment and bath chemistry

Small uncoated areas—often only a few millimeters—almost always trace back to surface prep or process errors. ISO 1461 explicitly excludes tiny bare spots (≈2–5 mm) from rejection, noting that sacrificial zinc nearby still protects such minor areas (ckit.co.za). Root causes include leftover weld spatter, embedded debris or rust from incomplete cleaning, and coatings blocked by lifting wires or mill scale (studylib.net) (ckit.co.za).

Residual oil, grease, or oxide films left by degreasing/pickling prevent zinc bonding (agindo.org) (studylib.net). If the flux layer (zinc chloride–ammonium chloride salt film that keeps fresh oxide off before dipping) dries out—because of long delays or overheating before the dip—its iron phosphate film breaks down (“over‑drying”), inviting rust and inhibiting coating (ckit.co.za). In the bath itself, excess aluminum above ~0.007% produces “black spots” (bare areas) (ckit.co.za). Physical barriers such as hooks/chains or sand trapped in castings leave local bare patches (studylib.net) (fabmann.com).

Remedies in practice include thorough cleaning (SP1 solvent degreasing, followed by complete rinse) and complete pickling so no mill scale remains. Flux needs to be fresh and in range—2–4% ammonium chloride—and not over‑dried; heated, prompt transfer from flux to zinc keeps the flux film active (ckit.co.za) (studylib.net). Design hanging points to avoid contact marks. Very small spots (<~5 mm) are usually benign; larger discontinuities require strip and re‑galvanizing (ckit.co.za) (studylib.net). Accurate chemical dosing supports flux control (see dosing pumps), and complete rinsing is eased by standard plant utilities (see supporting equipment for water treatment).

Excessive thickness: steel chemistry and time‑temperature

Target zinc thickness depends on steel thickness; ASTM A123 Table 1, for example, requires 45–100 µm depending on the category (bondigalva.co.id). Coatings hundreds of microns thick usually reflect material and process issues. Silicon and phosphorus in the steel act as catalysts for rapid Zn–Fe alloy growth; steels with ~0.04–0.15% or >0.22% Si (the “Sandelin range”) and P>0.04% form unusually thick, matte coatings (fabmann.com) (azom.com).

Longer immersion times or higher bath temperatures also drive thickness. Batch loads typically soak 3–10 minutes, longer for heavy loads, so over‑immersion or “over‑soaking” overshoots intent (azom.com). Inadequate drainage—especially from deep holes or tight threads—or failing to air‑knife excess zinc leaves metal build‑ups. Very thick alloy layers (≈≥200 µm) become brittle and can delaminate from the steel (azom.com).

Controls include selecting steel chemistries with Si and P in recommended bands (fabmann.com), precise dip timing, and high‑pressure air knives or centrifuges on threaded parts. Monitor coating weight using ASTM tables (as adopted in SNI 07‑7033) to check minimum µm by steel gauge; for example, 6–13 mm plate must get ≥85–100 µm zinc, and much more than this serves little benefit and risks flaking (bondigalva.co.id) (azom.com).

Adhesion failures: brittle alloys and contamination

True adhesion failure—coating peeling or flaking—is rare because zinc metallurgically fuses to steel. Failures typically involve special conditions. Thick‑coating scenarios on high‑Si/P steels produce brittle alloy layers that crack and separate under thermal or mechanical stress (azom.com) (fabmann.com). Excessive cooling stresses—such as quenching very thick, rigid coatings—can induce interface cracks (azom.com).

Contamination matters too. Passivated surfaces prevent bonding; chromate coatings (sometimes used in quench tanks for “wet storage stain” protection) must be stripped before galvanizing (kta.com). Any oil/wax film left from fabrication can cause lifting. Neutral process controls include solvent degreasing and verification for residual chromate (e.g., CuSO₄ test per SSPC‑SP16) (kta.com). Quenching practice also counts; a mild alkali quench, rather than plain water spray, minimizes thermal shock. If peeling is observed, it usually points to hydrogen embrittlement or a steel defect, warranting investigation into steel grade and handling. Very thick coatings (>0.2 mm) are avoided because they are prone to spalling (azom.com).

Pre‑treatment controls: cleaning, pickling, fluxing

Cleaning/degreasing removes grease, oil, and wax that block zinc bonding; guides emphasize that steel will not react with zinc to form a coating unless the surface is completely clean (agindo.org) (studylib.net). Pickling (acid cleaning with HCl/H₂SO₄) removes rust and mill scale; under‑pickling leaves scale or nitride films that cause localized bare spots, while over‑pickling risk (“acid burn”) is mitigated by prompt water rinsing.

Fluxing (zinc chloride–ammonium chloride) lays down a thin salt film to prevent oxide before dipping. Concentration that is too low, aged flux, or parts that dry after fluxing (over‑drying) lead to bare spots; conversely, excess flux residue can entrap as flux inclusions, a rare cause of white rust (ckit.co.za). [Holding flux at 2–4% ammonium chloride](https://beta.co.id/en/blog/galvanizers-are-stopping-the-drain-inside-the-chemistry-extending-flux-bath-life) and avoiding over‑drying are central controls. Many shops formalize this with chemical dosing control (see dosing pumps) and by standardizing rinse utilities (see supporting equipment for water treatment).

Process setpoints: bath and time

Bath temperature and immersion time steer both thickness and adhesion. AGI notes the zinc kettle should run ~450–460°C for normal reaction (agindo.org). If bath aluminum or other additives drift out of range, defect rates climb. Immersion time should match load weight; short dips yield thin spots, and over‑dipping yields excessive alloy and potential adhesion problems (azom.com) (azom.com).

Inspection, standards, and lifetime math

ASTM A123/SNI specify minimum µm targets by steel gauge, and deviations beyond typical shop thresholds (±20–30%) trigger repairs (bondigalva.co.id). Life expectancy is roughly linear with coating thickness; increasing from ~50 µm to 150 µm can mean decades longer until first maintenance (galvanizeit.org) (fabmann.com). Data‑driven inspection—checking coating thickness with gauges and visually inspecting for <5 mm gaps—catches defects early and guides rework or design fixes before large‑scale failures (ckit.co.za).

Sources and cross‑references

Indonesian regulations (SNI 07‑7033) reference ASTM A123/ISO 1461 norms (bondigalva.co.id). Industry guides (HDGASA, AGA) document common defects and causes (ckit.co.za) (studylib.net). Metallurgical studies and trade documents explain steel chemistry effects (fabmann.com) (azom.com). Galvanizing associations document thickness–life relationships (galvanizeit.org) (fabmann.com). Specific defect mechanisms—welding slag, sand inclusions, excess aluminum—are also cataloged (studylib.net) (ckit.co.za), and AGA/AGI note setpoints such as the 450–460°C bath range and the need for perfectly clean steel (agindo.org). All claims above are backed by these sources.