A 5 µm coating and a 25 µm coating are indistinguishable to the touch - you cannot tell with the naked eye whether a part got a layer that will protect it for years or one that will give up after a single season. The whole difference plays out on the micrometer scale and in whether someone actually measured the thickness or merely declared it. In this article we explain how standards translate zinc thickness into real corrosion resistance, what methods are used to check it, and what (besides the number of micrometers itself) decides whether a coating actually holds up.
How an Electroplated Coating Protects Steel - Barrier and Cathodic Protection
Electroplated coating thickness is the number of micrometers of zinc deposited on the steel surface - at the Strumet electroplating shop from 3 to 25 µm, depending on the class and intended use of the part. This parameter is the main factor determining corrosion resistance. Zinc protects steel cathodically (it "sacrifices itself" as the anode before corrosion reaches the substrate) and as a barrier, and the thicker the layer, the longer this protection lasts.
It is worth pointing out right away the difference from hot-dip galvanizing, where the coating is several times thicker (usually 50-100 µm) and is formed by immersion in molten zinc rather than by electrolysis. That is, however, a separate topic, to which we 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 the ISO 2081:2025 standard (in Poland PN-EN ISO 2081). It organizes the requirements for electrodeposited zinc coatings into a system of thickness designations based on the Fe/Zn symbol followed by a number specifying the minimum thickness in micrometers. Thanks to this, instead of describing requirements in words, it is enough to give a single designation in the order - e.g. Fe/Zn12 - and the contractor knows exactly what layer to deposit.
| Class | Thickness [µm] | Typical use |
| Fe/Zn5 | ≥ 5 | Dry interiors, heated rooms |
| Fe/Zn8 | ≥ 8 | Interiors with moderate humidity |
| Fe/Zn12 | ≥ 12 | Covered spaces, light outdoor exposure |
| Fe/Zn25 | ≥ 25 | Aggressive environments, atmospheric exposure |
The number in the designation is the minimum local thickness - a value that must be met at the least-coated point of the significant surface, not the average for the whole part. That is why thickness control only makes sense when it is carried out at many points, not at one "convenient" spot.
The general rule is simple: the more demanding the service conditions, the higher the class and the thicker the coating. For fasteners - bolts, nuts and washers - a separate ISO 4042 standard applies, which takes into account the specifics of threads (a coating that is too thick could make it impossible to tighten the joint) and the risk of hydrogen embrittlement. For parts of high hardness (hardened steels, strength classes 10.9 and higher), hydrogen embrittlement relief baking is also important, that is, heat treatment of the components after zinc plating - the treatment removes the hydrogen introduced during the process and prevents the part from cracking under load.
From C1 to CX - How the Aggressiveness of the Environment Determines the Required Thickness
The choice of Fe/Zn class is not arbitrary - it follows directly from how aggressive the surroundings are. Atmospheric corrosivity categories are defined by the standard ISO 12944-2:2017, which divides environments from C1 (heated interiors, negligible corrosion), through C3 (cities, moderate industry) and C4 (production plants, coastal zones), up to C5 (heavy industry, marine areas) and the CX category added in the revision (extreme environments, e.g. offshore installations).
The key to understanding the choice of thickness is the corrosion rate of zinc. In a given environment, zinc is lost at a more or less constant annual rate, so the service life of the coating can be roughly estimated as the thickness divided by this rate. The problem is that the rate rises sharply between individual corrosivity categories - according to ISO 12944-2 the annual zinc loss is approx. 0.1-0.7 µm in category 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 enough, but in a production hall (C4) the same layer would disappear many times faster. So to maintain the same durability, zinc electroplating must deliver a correspondingly thicker coating.
Coating Thickness Measurement - Which Method to Choose and When?
Declaring a class is one thing, and confirming it is another. Coating thickness is measured using several techniques, selected according to the stage of the process and the geometry of the part.
| Method | Principle of operation | Impact on the part | Accuracy | Typical use |
| Magnetic (ISO 2178) | Measuring the change in magnetic field over a non-magnetic coating on steel | Non-destructive | Good | Routine production control |
| Metallographic (ISO 1463) | Measuring the layer on a cross-section specimen under a microscope | Destructive | Highest (reference) | Dispute resolution, nonconformity investigations |
| X-ray XRF (ISO 3497) | Analysis of secondary radiation excited in the coating | Non-destructive, contactless | Very good | Parts with complex geometry, small measurement areas |
The magnetic method, described in the ISO 2178:2016 standard, is fast and does not damage the part, which is why it is the standard for ongoing shop-floor control. The microscopic (metallographic) method is destructive - it requires cutting and mounting the sample - but it gives the reference result when a doubt has to be settled unambiguously. XRF, in turn, works where the magnetic probe cannot sit flush on the surface: on edges, in recesses and on parts with a complex shape. A supplement is sometimes the coulometric method (ISO 2177), based on controlled anodic dissolution of the coating - particularly useful for thin electrodeposited coatings.
Zinc Plating Quality Control, or Why an In-House Laboratory Matters
In an electroplating shop with its own laboratory, zinc plating quality control is not a one-off test at the end, but a series of checkpoints spread across the whole process. It starts with surface preparation and bath composition, through thickness measurements on representative parts from successive loads, up to checking passivation and appearance before packing. In practice this means:
- In-process measurements - checking thickness during a run rather than only after it ends, which allows parameters to be corrected on an ongoing basis.
- Measurements at many points on the part - because the coating distribution is not perfectly uniform, and the standard specifies the minimum at the least-coated spot.
- Documentation of results - the customer receives documented values confirming compliance with the ordered class.
- Detecting nonconformities before shipment - a part outside tolerance goes back for reprocessing instead of to the customer.
This diligence has measurable value: a well-documented process also makes it possible to learn from difficult cases. This is shown, for example, by our case study on the effect of a third anode on zinc plating quality, where a change in anode configuration clearly improved coating uniformity.
Thickness Is Not Everything - What Else Determines Coating Durability?
Thickness is a necessary but insufficient condition. The actual service life of the protection is determined by several interacting factors:
- Current density and its distribution on the part - in areas of high current density the coating builds up faster, and in recesses more slowly; uneven distribution means thin, poorly protected zones.
- Electroplating bath composition - zinc concentration, additives and electrolyte purity translate directly into the structure and adhesion of the layer.
- Quality of surface preparation - degreasing and pickling of the steel remove contaminants and oxides; without this, even a thick coating will flake.
- Passivation and sealing - an additional conversion layer applied after zinc plating can extend the time to the appearance of corrosion many times over at the same zinc thickness.
- Part geometry - gaps, sharp corners and recesses create the so-called Faraday cage effect, in which current (and therefore zinc) barely reaches the interior.
How to Order the Right Coating Thickness? Questions Worth Asking the Contractor
Specifying requirements does not have to be complicated. The customer should give the following information in the order: a reference to the standard and class, matched to the type of passivation (e.g. "ISO 19598, Fe/Zn12" for the chromium-free variant or "ISO 2081, Fe/Zn12" for Cr(VI) passivation) and - if possible - the target corrosivity category of the part's service environment. On this basis the electroplating shop will select process parameters so that the minimum electroplated coating thickness is met even in the least accessible places.
It is also worth knowing how pricing works. For zinc electroplating, price per kg is the most common model, although the cost depends not only on the weight of the load but also on the ordered coating thickness and part geometry (components with a complex shape require more careful racking and a longer process). Specifying requirements precisely at the inquiry stage helps avoid misunderstandings and get a quote that matches the real scope of work.
If you need to choose the right class for your application or simply get a quote for a specific batch, contact the sales department of our electroplating shop - we will help match the thickness and process parameters to the real service conditions of your parts.
FAQ - Most Frequently Asked Questions About Electroplated Coating Thickness
What is the minimum thickness of an electroplated zinc coating for components used outdoors?
For atmospheric exposure, classes from Fe/Zn12 upward are usually used, and in aggressive environments (marine, industrial) Fe/Zn25 or more. In extreme conditions - category CX according to ISO 12944-2, e.g. offshore installations - thicker coatings or duplex systems are often chosen. The final value should follow from the corrosivity category of the environment.
Does a thicker zinc coating always mean longer protection against corrosion?
As a rule, yes - thicker zinc means a longer-lasting barrier. It will not, however, replace good surface preparation, passivation and an even distribution of the coating, which also determine durability.
How is the thickness of an electroplated coating measured?
The non-destructive magnetic method per ISO 2178 is most commonly used, and if needed the result is verified by the metallographic method on a cross-section or by X-ray XRF for parts with complex geometry.
What is the difference between measuring coating thickness with the magnetic method and with the microscopic method?
The magnetic method is fast and non-destructive, so it is used for routine production control. The microscopic method requires cutting the part but gives a reference result of the highest accuracy.
How can you check whether a zinc plating contractor actually controls coating thickness?
Ask for the measurement documentation for your batch and ask what methods are used and at how many points the control is carried out. A contractor with its own laboratory will readily present documented results consistent with the ordered class.
