White Rust on Zinc Die Castings: Causes and Prevention

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A batch of zamak components leaves the warehouse, or arrives after weeks at sea, and when the cartons are opened the parts are covered in a white, powdery deposit, often concentrated exactly where two surfaces were touching. This is white rust on zinc die castings, also known as wet storage stain. In almost every case it is not an alloy defect: it is caused by moisture and condensation during storage and transport, and it is largely preventable.

Below we look at the chemistry of the deposit, its logistics-related and plating-related causes, how to tell it apart from other defects, the role of passivation, packaging rules, verification tests and remedies. We write from a foundry’s perspective: Micrometal has been producing zinc die castings in Erbusco (Brescia, Italy) since 1991, supplying customers across Europe and beyond.

What white rust on zamak is: chemistry and appearance

Why “oxidation” is a misleading term

The deposit is not pure zinc oxide. Technical literature on wet storage stain describes it as a mixture of basic zinc carbonate 2ZnCO₃·3Zn(OH)₂, zinc oxide ZnO and β-Zn(OH)₂ hydroxide. It is easy to recognise: white, bulky and crumbly, occupying roughly 100 times the volume of the zinc consumed. That is why it looks so dramatic even when metal loss is minimal.

Protective patina or wet storage stain

Oxygen and carbon dioxide make the difference. A zamak surface exposed to air, with O₂ and CO₂ available, develops a thin, compact and protective carbonate patina. If, instead, a film of water is trapped between parts in contact with poor air circulation, the patina cannot form and the bulky deposit grows: this is wet storage stain.

How serious is it?

According to the International Zinc Association (IZA), atmospheric corrosion of zinc die castings rarely exceeds about 12 µm per year and can be as low as one tenth of that figure; on a solid casting, white rust is more unsightly than harmful (general reference: ASM Handbook Vol. 13, Corrosion). For decorative parts and for parts still awaiting finishing, however, it is a real cause of rejection.

The main causes: moisture, condensation and temperature cycling in storage and transport

Condensation from temperature swings. IZA is unambiguous: dry gases (O₂, N₂, CO₂) do not corrode zinc; liquid water is required. The typical source is condensation: day/night temperature swings, unheated warehouses, and cartons moving from a cold environment into a warm, humid one.

Stacked, nested or touching parts. Water gets trapped while air circulation and CO₂ are lacking: ideal conditions for wet storage stain. Small hot chamber die castings, often packed loose in large quantities, are precisely the case where packaging and storage determine the outcome.

Sea freight. Industry practitioners, in technical discussions on the finishing.com forum, report cyclic condensation patterns and traces of salt spray even in containers considered “sealed”. This is field experience rather than normative data, but it is consistent with the mechanism.

Accelerators. Chlorides and sulphates speed up the formation of the deposit: marine environments, contaminated rinse water, washing residues. Add to this fingerprints on passivated parts and incomplete drying after vibratory finishing or washing.

Typical behaviour of the alloys. In humidity tests carried out under the ZCA programme (10 days at 95 °C in a humid environment), Zamak 3 and Zamak 5 castings developed white rust, described as typical behaviour of foundry alloys exposed to moisture. The ZP3, ZP5, ZP2 and ZP8 zamak alloys differ mainly in their mechanical properties: where moisture is concerned, prevention has to be managed, not taken for granted.

When the problem starts in the plating shop: porosity, pre-cleaning and the copper layer

Not every white bloom originates in the warehouse. On nickel-, gold- or silver-plated parts, stains or blisters that appear weeks after finishing often have their roots in the plating cycle.

Surface porosity and entrapped solutions. Open surface pores can retain bath residues, which over time bleed out as white stains or lift the coating.

Insufficient pre-cleaning. According to plating practitioners (finishing.com), porosity and inadequate pre-cleaning (oils, release agents, residual oxides) are the main causes of blistering of Cu-Ni coatings on zinc die castings.

The plating sequence on zamak. The typical cycle starts with a copper base layer (cyanide copper strike, indicatively at least 5 µm according to ASTM B252), followed by acid copper, then nickel and finally the decorative finish. The copper base is essential: if it is too thin, the acid copper solution reaches the zinc through the pores and attacks it. Cu-Ni-Cr coatings on zinc alloys are covered by EN ISO 1456 (which replaced the withdrawn EN 12540). A sound base layer is the prerequisite for both gold plating on zamak and silver plating of zamak parts.

The foundry’s role. Plating quality starts with the casting: hot chamber zinc die casting run with controlled parameters produces a compact, low-porosity skin (general reference: NADCA Product Specification Standards for Die Castings). Micrometal operates 11 units, 7 hot chamber die casting machines and 4 robotic cells, from 20 to 90 t; electroplated finishes are carried out by qualified plating partners, coordinated by Micrometal on the components it casts.

Raw castings awaiting finishing. If white rust forms before plating, it must be removed during pretreatment; otherwise it will compromise adhesion.

White rust, tarnishing and intercrystalline corrosion: how to tell them apart

When faced with a “damaged” part, the first question is: is this a surface problem or a material problem? According to IZA, zinc die castings used indoors simply lose some of their lustre over time. Intercrystalline corrosion (zinc pest) is a different phenomenon altogether: swelling, cracking and loss of strength caused by impurities such as Pb, Sn and Cd above the permitted limits. IZA states it plainly: castings that comply with EN 12844 are not subject to it. Ingot composition is governed by EN 1774; in the US, the reference for die castings is ASTM B86.

Micrometal works with the four alloys ZP3, ZP5, ZP2 and ZP8 in accordance with EN 12844, under an ISO 9001 certified quality system. With compliant alloys, intercrystalline corrosion is ruled out: a white bloom should be treated as a surface and storage problem.

Trivalent Cr(III) passivation: how it works, performance and limitations

How it works. Passivation forms a thin conversion layer (reference for chromate conversion coatings: ISO 4520) that slows down moisture attack on the zinc surface. It slows it down; it does not stop it. It is not an absolute barrier.

Cr(III) versus Cr(VI). The RoHS Directive 2011/65/EU limits Cr(VI) to 0.1% in homogeneous materials, and the REACH Regulation (EC) 1907/2006 makes its compounds subject to authorisation. Trivalent chromium conversion coatings are far less toxic and offer equal or better corrosion resistance, often with the help of a sealer or top coat.

Indicative performance. For passivated die castings, IZA reports around 240 h in ASTM B117 salt spray as readily achievable, and up to about 400 h; clear and black versions generally fall below that. These values depend on the chemistry of each specific product, which is largely proprietary.

Practical limitations. Colour varies with the alloy; no simple dip treatment withstands 120 °C without a significant loss of protection; fingerprints and handling degrade the layer.

When to choose it. Passivation makes most sense for parts that will not be plated and for technical functions where tolerances and appearance matter; a sealer or lacquer adds margin for long storage periods or cosmetic requirements. Passivation is one of the finishes Micrometal coordinates with qualified partners: performance must be defined and verified on the specific part.

Packaging and storage: the most effective prevention

If zinc only corrodes in the presence of liquid water, the main lever is to stop water from reaching the parts and staying on them. Most cases are prevented right here.

Complete drying after washing, vibratory finishing and plating: no damp part should ever be packed. Packaging with VCI film or bags, desiccant sachets sized to the pack volume, and dividers or cell inserts to prevent nested parts and touching surfaces. For sea freight and long storage periods, the practice recommended by industry practitioners is full wrapping with no openings, plus container desiccants and humidity indicators.

In the warehouse, you need a dry, ventilated area away from loading doors and sources of temperature swings, pallets raised off the floor, and FIFO stock rotation. When handling, wear gloves on passivated parts or parts destined for finishing; on arrival, let cartons acclimatise before opening them to avoid condensation on cold parts.

An ISO 9001 management system makes it possible to turn all of this into a documented requirement, traceable by batch. At Micrometal, packaging is normally carried out in-house, so it pays to define the packaging specification at the quotation stage. If your parts face long storage periods or sea freight, request a technical assessment.

Verification tests: ISO 9227 salt spray and ISO 6270-2 condensation testing

Neutral salt spray (ISO 9227, US equivalent ASTM B117) is a comparative accelerated test: useful for comparing finishing cycles, not for predicting service life. The condensation atmosphere test (ISO 6270-2), on the other hand, reproduces the condensation that causes white rust during storage and transport, so it is closer to the real-world problem.

On the drawing or in the specification, it is worth stating: the acceptance criterion (first appearance of white corrosion products or substrate corrosion), the number of samples, and the condition of the part (raw, passivated, coated). Results depend on the passivation chemistry and on the geometry: only compare cycles tested under the same conditions.

What to do if white rust appears: assessment, cleaning and rework

Before deciding, ask three questions: how extensive is the deposit (a light haze or a thick layer), what kind of part is it (raw, passivated, plated), and what is its function (decorative or technical)? IZA points out that the phenomenon is mainly cosmetic, so the decision depends on how the component is used.

The last step is the most important: without tracing the root cause, the problem will come back with the next batch. Micrometal has been producing zinc die castings in Erbusco since 1991, with an ISO 9001 certified quality system. If you are evaluating a component that needs protection during storage or is destined for electroplating, contact our technical team or call +39 030 7760830.

Technical review: Marco Sega

Content produced with the assistance of artificial intelligence systems and subject to technical oversight by our editorial team. Editorial responsibility: Micrometal S.r.l.