Coatings 5 and 25 µm thick are indistinguishable under the finger – with the naked eye, you can't tell whether a detail has a layer that will protect it for years or one that will give out after a single season. The entire difference lies in the micrometer scale, and whether someone actually measured this thickness or just declared it. In this article, we explain how standards translate zinc thickness into actual corrosion resistance, what methods are used to check it, and what (besides the micrometer count itself) determines whether a coating actually holds.

How Galvanic Coating Protects Steel – Barrier and Cathodic Protection

The thickness of the galvanic coating is the number of micrometers of zinc deposited on the steel surface – in Strumet electroplating plant from 3 to 25 µm, depending on the class and purpose of the detail. This parameter is the main factor determining corrosion resistance. Zinc protects the steel cathodically ("it sacrifices itself" as an anode before corrosion reaches the substrate) and barrierly, and the thicker the layer, the longer the protection lasts.

It's worth noting the difference from hot-dip galvanizing, where the coating is several times thicker (usually 50–100 µm) and is created by immersion in molten zinc, not electrolysis. However, this is a separate topic, to which we have devoted a separate article: Zinc coating – characteristics, process and durability.

Zinc coating standards: ISO 2081 and Fe/Zn classes in practice

The basic document describing electrolytically applied zinc coatings is standard ISO 2081:2025 (in Poland PN-EN ISO 2081). It organizes the requirements for electrolytic zinc coatings into a system of thickness designations based on the Fe/Zn symbol, along with a number specifying the minimum thickness in micrometers. This means that instead of describing the requirements verbally, a single designation—e.g., Fe/Zn12—is sufficient in the order, allowing the contractor to know exactly which layer to deposit.

The number in the designation is minimum local thickness – a value that must be maintained at the least covered point of the significant surface, not an average across the entire part. Therefore, thickness control only makes sense when performed at multiple points, not at a single, "convenient" location.

The general rule is simple: the more demanding the operating conditions, the higher the class and thicker the coating. Fasteners – screws, nuts, and washers – have a separate ISO 4042 standard, which takes into account the specific nature of the threads (too thick a coating could prevent the connection from being screwed together) and the risk of hydrogen embrittlement. For high-hardness details (hardened steels, strength classes 10.9 and higher), it is also important dehydrogenation, i.e. heating the elements after galvanizing – the treatment removes the hydrogen introduced in the process and prevents the detail from cracking under load.

From C1 to CX – how environmental aggressiveness determines the required thickness

The choice of Fe/Zn class is not accidental – it results directly from how aggressive the environment is. The atmospheric corrosivity categories are defined standard ISO 12944-2:2017, dividing environments from C1 (heated interiors, negligible corrosion), through C3 (cities, moderate industry) and C4 (production plants, coastal zones), up to C5 (heavy industry, coastal areas) and the category CX (extreme environments, e.g. offshore installations) added in the amendment.

The key to understanding thickness selection is the zinc corrosion rate. In a given environment, zinc depletes at a roughly constant annual rate, so the coating's service life can be roughly estimated as thickness divided by this rate. The problem is that the rate increases abruptly between each corrosivity classes – according to ISO 12944-2 the annual zinc loss is approximately 0.1–0.7 µm in class C2, 0.7–2.1 µm in C3 and 2.1–4.2 µm in C4. This means that for a dry office (C2), a thin coating of a few micrometers is sufficient, but in a production hall (C4), the same layer would disappear many times faster. Therefore, to maintain the same durability, electrogalvanizing must provide a correspondingly thicker coating.

Coating thickness measurement – which method to choose and when?

Declaring the class is one thing, confirming it is another. Coating thickness measurement is performed using several techniques, selected depending on the process stage and the geometry of the part.

Magnetic method, described in the standard ISO 2178:2016, is fast and does not damage the part, which is why it is the standard for ongoing inspection in the shop floor. The microscopic (metallographic) method is destructive – it requires cutting and cross-sectioning the sample – but it provides a reference result when a doubt needs to be clearly resolved. XRF, on the other hand, is useful where the magnetic probe cannot adhere to the surface: on edges, in recesses, and on details with complex shapes. It can be supplemented by coulometric method (ISO 2177), based on controlled anodic dissolution of the coating – especially useful for thin electrolytic coatings.

Galvanizing quality control, or why having your own laboratory is important

In a galvanizing plant with its own laboratory, galvanizing quality control is not a one-time test at the end, but a series of checkpoints distributed throughout the entire process. This begins with surface preparation and bath composition, proceeding through thickness measurements on representative parts from subsequent batches, and ending with passivation and appearance inspections before packaging. In practice, this means:

• Inter-operational measurements – checking the thickness during the series, not only after its completion, which allows for correcting parameters on an ongoing basis.
• Measurements at multiple points of the detail – because the distribution of the coating is not perfectly even, and the standard specifies a minimum in the least covered place.
• Documentation of results – the customer receives documented values that confirm compliance with the ordered class.
• Pre-shipment discrepancy detection – a part outside the tolerance is sent for re-processing and not to the recipient.

This diligence has tangible value: a well-documented process also allows for learning from difficult cases. This is demonstrated by our case study on the influence of the third anode on the quality of galvanizing, where changing the anode configuration significantly improved the coating uniformity.

Thickness is not everything – what else determines the durability of the coating?

Thickness is a necessary, but not sufficient, condition. The actual lifespan of the protection is determined by several interrelated factors:

• Current density and its distribution on the workpiece – in areas of high current density the coating grows faster and in recesses slower; uneven distribution means thin, poorly protected zones.
• Composition of the galvanic bath – zinc concentration, additives and electrolyte purity directly translate into the structure and adhesion of the layer.
• Quality of surface preparation – degreasing and pickling steel removes impurities and oxides; without this, even a thick coating will flake off.
• Passivation and sealing – an additional conversion layer applied after galvanizing can extend the time until corrosion occurs many times with the same zinc thickness.
• Detail geometry – gaps, sharp corners and recesses create the so-called Faraday cage effect, in which the current (and therefore zinc) does not reach the interior well.

How to Order the Right Coating Thickness? Questions to Ask Your Contractor

The requirements specification doesn't have to be complicated. The client should provide the following information in the order: reference to standard and class, matched to the type of passivation (e.g. "ISO 19598, Fe/Zn12" for chromium-free variant or "ISO 2081, Fe/Zn12" for Cr(VI) passivation) and – if possible – target corrosivity class of the work environment of the detail. Based on this, the electroplating facility will select process parameters to ensure the minimum thickness of the galvanic coating is maintained even in the most inaccessible areas.

It's also worth knowing how the billing works. In the case of electroplating, the price per kg is the most common model, with the cost depending not only on charge weight, but also from the ordered coating thickness and detail geometry (Complex elements require more careful hanging and a longer process.) Defining your requirements precisely at the inquiry stage helps avoid misunderstandings and ensure you receive a quote that reflects the actual scope of work.

If you need to select the right class for your application or simply request a quote for a specific batch, contact the sales department of our galvanizing plant – we will help you match the thickness and process parameters to the actual operating conditions of your parts.

FAQ – most frequently asked questions about the thickness of the galvanic coating

What is the minimum thickness of the galvanic zinc coating for components used outdoors?

For atmospheric exposure, classes of Fe/Zn12 or higher are typically used, and in aggressive environments (coastal, industrial) Fe/Zn25 or higher. In extreme conditions – class CX according to ISO 12944-2, e.g., offshore installations – thicker coatings or duplex systems are often used. The final value should be based on the environmental corrosivity class.

Does a thicker zinc coating always mean longer corrosion protection?

In principle, yes – thicker zinc provides a longer-lasting barrier. However, this does not replace good surface preparation, passivation, and even coating distribution, which also determine durability.

How to measure the thickness of a galvanic coating?

Most often, the non-destructive magnetic method according to ISO 2178 is used, and if necessary, the result is verified by the metallographic method on a cross-section or by the XRF X-ray technique for details with complex geometry.

What is the difference between measuring coating thickness using the magnetic method and the microscopic method?

The magnetic method is fast and non-destructive, making it suitable for routine production inspection. The microscopic method requires sectioning the part, but provides a reference result with the highest accuracy.

How can I check if the galvanizing contractor actually controls the coating thickness?

Request measurement documentation for your batch and ask what methods and at what points they perform the inspection. A supplier with its own laboratory will easily provide documented results consistent with the ordered grade.