Zinc flake coating

Key takeaways

Zinc flake coating belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Zinc flake coating to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Zinc flake coating from memory before moving on to harder problems.

Reference excerpt

Zinc flake coatings are non-electrolytically applied coatings, which provide good protection against corrosion. These coatings consist of a mixture of zinc and aluminium flakes, which are bonded together by an inorganic matrix. The specifications for zinc flake coatings are defined in international standard ISO 10683 and also in European standard EN 13858. ISO 10683 sets out the requirements for zinc flake coatings for threaded fasteners and EN 13858 describes the requirements for zinc flake coatings for fasteners with no thread and for other parts as well. There are three groups of zinc flake coatings:

Zinc flake coatings containing Cr(VI) (hexavalent chromium): surfaces containing Cr(VI) provide greater anti-corrosion protection with a thinner coating, but Cr(VI) is carcinogenic and poses a potential risk to the environment. Zinc flake coatings containing Cr(VI) are no longer available in Europe since June 2017. Solvent-based Cr(VI)-free zinc flake coatings. Water-based Cr(VI)-free zinc flake coatings. Cr(VI)-free coatings are more environmentally friendly than surfaces with a Cr(VI) content. No zinc flake coatings used in the automotive industry nowadays contain this substance. Various manufacturers, such as car companies and their suppliers, have produced their own specifications and supply rules in order to define the requirements for these coating systems. Zinc flake coating is a generic term for the coating technology and this is marketed by the different suppliers under their respective brand names.

History Because electrolytically zinc-plated surfaces provide comparatively little corrosion protection, and in the case of galvanic zinc coatings on high-strength steel (e.g. category 10.9 and 12.9 high-strength bolts) there is a risk of hydrogen embrittlement, the industry needed a better corrosion protection system. High-strength steel parts (such as bolts of strength category > 10.9 and nuts of > 9) and components with tensile strength of > 1000 N/mm2 or > 320 HV are susceptible to hydrogen embrittlement. Galvanic coating processes and pickling with acids have a major influence on the development of hydrogen-induced brittle fractures. In the 1970s, a new coating system was developed in the United States: zinc flake coating (patent number 1376067). By virtue of a thin coat thickness of typically 8-12 μm this system produced a high level of protection against corrosion and made it possible to avoid hydrogen embrittlement. In the 1980s and 1990s, the use of these coating systems spread, e.g. within the automotive industry. This industry needs coating systems offering a high level of corrosion resistance. As zinc flake coatings do not create any hydrogen in the process, they were used for critical applications as an alternative to electroplating.

Characteristics Today, these are the preferred coatings for fasteners and other parts in the automotive industry, as they offer various advantages:

Good appearance (colouring) Very good protection against corrosion (240 - 1,500 hours, depending on specification and coating thickness) Resistant to extreme temperatures Good chemical resistance Environmentally friendly Good friction characteristics (on bolts and nuts) No warm-loosening torque No risk for high-strength fasteners of any hydrogen embrittlement Electric conductivity Other assembly properties The requirements of the automotive industry have set new objectives. Corrosion protection and appearance are no longer the only and most important characteristics. In addition to the applications in the automotive industry, these coating systems are also found in wind power systems, the construction industry, electrical equipment (plant construction), trucks and other markets as well. Zinc flake coatings create what is known as cathodic protection: the less noble zinc 'sacrifices' itself in order to protect the underlying metal. Steel can be protected in this way. The coating thickness is often between 5 μm and 15 μm, with thicker layers also possible where there are special requirements. When coating metric threaded parts it is necessary to keep to the tolerances defined in ISO 965 so that the bolt's thread does not get gummed up and the coefficient of friction can be set accordingly. Hot-dip galvanised fasteners with a typical coating thickness of 80-200 μm have to be grooved again retrospectively in order to expose the thread. In contrast to paints where the risk of sub-surface corrosion creep exists, this phenomenon is avoided through the sacrificial effect of the zinc. In salt spray tests zinc flake coatings demonstrate better protection against corrosion than a typical galvanic zinc coating, which in the tests (generally run in accordance with ISO 9227) often achieve only 96 to 200 hours.

Coating technique The material for the zinc flake coatings gets supplied in liquid form and needs to be prepared to the desired conditions before application. The viscosity, temperature and stirring time prior to application all play an important role here. The material can be applied using the following application techniques:

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Worked examples

Example 1 — a first encounter with Zinc flake coating

Start with the simplest possible case. Write down what Zinc flake coating claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Zinc flake coating before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Zinc flake coating ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Zinc flake coating

In research: Zinc flake coating appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Zinc flake coating in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Zinc flake coating is common in secondary-school and first-year university syllabi. It links to neighbouring topics Corrosion prevention, Zinc, so understanding it makes those chapters shorter.
In everyday life: Look for Zinc flake coating outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Zinc flake coating in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Zinc flake coating means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Zinc flake coating out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Zinc flake coating in simple terms?

Zinc flake coatings are non-electrolytically applied coatings, which provide good protection against corrosion. These coatings consist of a mixture of zinc and aluminium flakes, which are bonded together by an inorganic matrix.

Why does Zinc flake coating matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Zinc flake coating?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Zinc flake coating.