X-ray fluorescence control of the composition of aluminum-zinc and zinc coatings in rolled steel products

In the production of galvanized rolled steel products, it is critically important to strictly control the composition of metallic coatings. Modern protective coatings on steel – from traditional zinc to complex zinc-aluminum-magnesium alloys – contain alloying elements (additions) such as aluminum (Al), magnesium (Mg) and silicon (Si). These additions have a strong influence on the structure of the coating and its corrosion resistance, so accurate measurement of their concentrations is a necessary condition for ensuring quality.

X-ray fluorescence (XRF) analysis provides a fast, non-destructive way to determine the chemical composition of coatings directly in production. This article discusses the tasks that XRF solves in hot-dip galvanizing and steel rolling, how XRF is used for in-process production control, which tolerances and standards regulate coating composition, and gives examples of alloys (Zn-Al, Zn-Al-Mg, Zn-Mg) with their typical compositions and fields of application. Particular attention is paid to measuring Al, Mg and Si in zinc coatings as the key elements that determine the corrosion resistance of the material.

Tasks of XRF in hot-dip galvanizing and rolling

The X-ray fluorescence method is widely used at the stages of hot-dip galvanizing (continuous hot-dip or electrolytic coating of steel with zinc) and in rolling processes to solve a number of quality-control tasks:

Control of zinc bath composition.
On hot-dip galvanizing lines, alloying additions – for example aluminum and magnesium – are deliberately introduced into the molten zinc bath to obtain an alloyed coating. XRF makes it possible to quickly measure the content of these elements in the zinc melt and keep them within the specified range. For some zinc-aluminum-magnesium alloys, for example, production requires Al concentrations on the order of 2.0% ± 0.2% and Mg around 1.5% with a narrow tolerance. Maintaining this composition is critical: going outside the tolerance can deteriorate coating quality or adhesion. With XRF, technologists can regularly check bath samples and adjust the dosing of master alloys, ensuring a stable chemical composition.

Analysis of coatings on finished rolled products.
After applying coatings to rolled products (steel sheets, coils, wire, etc.), it is necessary to ensure that the actual composition of the layer obtained corresponds to the specification. XRF makes it possible to measure directly on the product sample the mass fractions of zinc, aluminum, magnesium, silicon and other elements in the coating without damaging the product. This is especially important for aluminum-zinc coatings (for example Zn-5%Al or Zn-55%Al) and the latest Zn-Al-Mg coatings, where even slight deviations in Al or Mg content significantly affect corrosion resistance.

In standard hot-dip galvanizing, for instance, the aluminum content is maintained at about 0.20–0.30% – enough to improve the adhesion of zinc to steel and to control growth of the intermetallic layer, but excessive Al is unacceptable. XRF provides accurate measurement even of such low Al concentrations, allowing both conventional galvanized coatings and complex alloy coatings to be controlled in accordance with standards.

Measurement of coating thickness and mass.
Besides chemical composition, XRF can also be used for indirect measurement of coating thickness based on the intensity of fluorescent lines of the elements. Special modes of XRF analyzers make it possible to calculate the thickness of a metallic layer (for example zinc on steel) using calibration with reference samples. This allows the mass of the applied zinc to be checked against required values (for example, 275 g/m² for Z275-class coating) without using destructive methods.

While magnetic thickness gauges are often used to measure thickness, XRF has the advantage of simultaneously providing both thickness and composition of the coating. This is particularly relevant for multi-component coatings: knowing the composition makes it possible to refine the thickness calculation and vice versa.

Control of phase composition and diffusion layers.
In the case of alloyed coatings or galvannealing, XRF makes it possible to assess the iron content in the coating. For example, in the production of a zinc–iron alloy (ZF coating) it is important to determine the Fe fraction (~8–12%) that has formed in the coating during diffusion. X-ray fluorescence is successfully used for such control of iron content, helping to tune furnace annealing parameters and obtain the desired alloy phase.

Thus, XRF is used both at the stage of coating application (on-line control of the bath and the primary layer at the exit from the pot) and at the final control of rolled products (checking a batch for compliance with the coating grade and standard). This method closes the critical gap between the production process and the quality of the finished product: analysis results are available within seconds, allowing rapid decisions on process adjustment or product release.

Control of Al, Mg and Si in zinc coatings

Aluminum, magnesium and silicon are three key alloying elements that may be present in zinc coatings and have a major impact on their properties. XRF plays a crucial role in accurately measuring the concentrations of these elements and ensuring the required coating characteristics:

Aluminum (Al).
Aluminum is added to zinc coatings primarily to enhance barrier properties and improve adhesion of the coating to steel. Even in conventional hot-dip galvanizing, a small Al addition (~0.2%) prevents excessive growth of a brittle zinc–iron layer at the steel–zinc interface, which ensures strong bonding of the coating.

In alloys such as Zn-5%Al (Galfan), aluminum significantly improves corrosion resistance by forming a fine eutectic zinc–aluminum structure that is more ductile and stable. An even higher Al level (~55%) in aluminum-zinc coatings (Aluzinc, Galvalume) creates a predominantly aluminum passive film on the surface, resulting in corrosion resistance in atmospheric conditions that is 2–6 times higher than that of conventional galvanizing of the same thickness.

At the same time, excess Al reduces the ability of the zinc coating to provide cathodic protection of exposed edges: on cut edges the steel remains less protected because aluminum in the alloy does not sacrifice itself as effectively as zinc. Therefore, Al content must be tightly controlled: each coating type has its own optimum (for example, about 5% in Galfan or about 55% in Galvalume). XRF makes it possible to measure aluminum with high accuracy even at fractions of a percent, which is necessary both for low-aluminum coatings (galvanizing, ~0.2% Al) and for high-aluminum ones (55% Al).

Magnesium (Mg).
Adding magnesium is a relatively new solution in zinc coatings aimed at sharply increasing corrosion resistance. Magnesium (typically about 1–4%) in alloy with zinc promotes the formation during corrosion of special protective compounds – for example basic zinc–magnesium salts that tightly seal the surface and prevent further rust development.

Studies show that even a small addition of Mg (~0.1–0.2%) to a Zn-5%Al alloy noticeably improves its corrosion resistance. At higher Mg contents (~3%) the effect becomes striking: for example, a Zn-3%Mg-3.5%Al coating (known as Magnelis®) forms a continuous stable passive layer over the entire surface and on cut edges, providing self-healing of scratches and protection of exposed edges several times more effective than a conventional zinc coating.

Magnesium also slightly increases the hardness and wear resistance of the zinc layer. However, the Mg content must stay within a strict window: too little will not give the desired increase in durability, while too much can cause problems with coating uniformity and solderability. Therefore, Mg control by XRF is standard practice on lines producing Zn-Al-Mg coatings.

Modern XRF analyzers can confidently detect magnesium (Z = 12) in alloys; for example, portable instruments with silicon drift detectors (SDD) register the Mg K line (~1.25 keV) even without vacuum, in air. This allows direct measurement of 1–3% Mg in coatings on steel in just a few seconds.

Silicon (Si).
Silicon is added to zinc alloys less frequently and usually in small amounts (typically ≤1.5%). Its main role is technological. In aluminum-zinc coatings (~55%Al–Zn), for example, about 1.5% Si is introduced to control the reactivity of the melt with the steel substrate. Silicon slows the growth of the Fe–Al–Zn intermetallic layer at the steel/coating interface, preventing formation of an excessively thick brittle layer.

As a result, Galvalume-type coatings are dense and adhere well despite their high aluminum content. In alloys with intermediate Al content (for example ~10% Al), a small Si addition (~0.2%) can also be used to improve wetting and the microstructure of the coating. Although silicon does not provide direct corrosion protection, controlling its concentration is important for process stability.

XRF successfully handles this task as well: the Si Kα line (~1.74 keV) falls in the same detection region as Al and Mg. Thus, one XRF measurement can determine the complete set of Zn, Al, Mg and Si in a multi-component coating.

By combining data on all key elements, XRF makes it possible to ensure that coating composition matches the optimum values at which the material delivers maximum corrosion resistance. The accuracy and repeatability of XRF (typical error is on the order of tenths of a percent) are sufficient to control tight tolerances on Al, Mg and Si in zinc alloys. All measurements are non-destructive – samples do not need to be etched or dissolved, which is especially valuable when controlling high-value rolled products.

Modes of using XRF: on-line express analysis and laboratory control

Organization of control using XRF can be flexibly integrated into the production process. Two main modes of using X-ray fluorescence analysis are applied:

On-line express analysis on the production line.
Portable handheld XRF analyzers (so-called “XRF guns”) make it possible to perform measurements directly on site, next to the galvanizing line or in the finished-goods warehouse. A quality engineer can simply place the instrument against the surface of a galvanized coil or selected sample and obtain composition results in 5–15 seconds.

Modern portable analyzers are compact and battery-powered, yet cover a wide range of elements – from Mg (No. 12) to U (No. 92). This means that the entire spectrum of required alloying elements (Al, Mg, Si and heavier elements) can be analyzed in situ with a single instrument. The speed of such control is extremely important: if, for example, measurements show Mg content outside allowable limits, corrective actions (adding Mg master alloy or diluting with zinc) can be taken immediately, without waiting for lengthy laboratory results.

Express XRF is also convenient for incoming inspection of raw materials and quality checks: when a batch of aluminum-zinc alloy ingots arrives, for instance, one can quickly confirm that its chemical composition meets the specification before charging the bath. On-site analysis also removes logistical delays – there is no need to cut samples and send them to a chemical laboratory, which can take hours. As a result, express XRF increases the responsiveness of quality control and reduces the risk of non-conforming product being shipped.

Laboratory control and calibration.
Despite the capabilities of portable instruments, some analyses are still carried out in the plant’s stationary quality laboratory. Bench-top energy-dispersive XRF spectrometers with even higher resolution and stability are typically used there. Laboratory XRF makes it possible to precisely calibrate the method for specific coating compositions using control samples and certified reference alloys. For a new type of coating, for example, a plant can develop its own XRF procedure and verify its metrological characteristics.

The laboratory also performs detailed analysis of questionable or critical samples – for instance, if a handheld analyzer has shown an atypical result or if there are disputes with a customer about coating quality. In addition, stationary instruments can determine ultra-low impurity levels (ppm range) that are not accessible in field conditions and can perform parallel analysis using standard methods (atomic emission spectrometry, wet chemistry) to confirm XRF results.

In practice, however, there is often no fundamental difference in accuracy between modern portable and laboratory XRF – many handheld models are equipped with the same SDD detectors and powerful X-ray tubes as laboratory systems. Their sensitivity is sufficient to detect Mg, Al and Si without a vacuum chamber or purge gas. Therefore, the main difference between express and laboratory modes lies more in the analysis conditions and goals (in-process control vs. reference analysis) than in elemental capabilities.

Combined use.
As a rule, on-line measurements on the line are complemented by periodic laboratory checks to confirm accuracy, and the results of stationary analyses are used to calibrate and adjust handheld instruments. This approach provides reliable monitoring of the galvanizing process at all stages.

Portable analyzers for Zn-Al-Mg coatings

Particular attention should be paid to modern portable XRF analyzers such as ProSpector 3, specially adapted for the analysis of complex coating alloys. These instruments represent a new generation of handheld spectrometers that combine high accuracy, speed and low detection limits for light elements. In the context of controlling Zn-Al-Mg coatings, such analyzers offer the following capabilities:

Reliable measurement of light elements.
ProSpector 3 is equipped with a highly sensitive SDD detector and advanced software corrections, which allow it to measure Mg, Al and Si in the presence of a large excess of Zn on a steel substrate. The instrument confidently registers the characteristic X-ray lines of Mg (about 1.25 keV), Al (~1.49 keV) and Si (~1.74 keV), and no vacuum chamber is required – analysis is performed in air thanks to the high sensitivity of the system. For production this means that even low concentrations of magnesium (~1%) or aluminum (<5%) in the coating will not go unnoticed.

Specialized coating modes.
The software of such analyzers includes Coating Mode, where the user can specify a multilayer sample model. During measurement the instrument automatically takes matrix effects into account: absorption of radiation in the coating, contribution of the iron substrate, and so on. As a result, the report provides both the chemical composition of the coating and, when calibration is available, its thickness.

For example, in thickness-measurement mode ProSpector shows a linear correlation between the measured XRF signal and the certified thickness of a Zn coating on steel. This makes it possible to evaluate the mass of the applied alloy (g/m²) simultaneously with its composition, which is extremely convenient for controlling coating weights on rolled products.

Speed and ease of analysis.
New-generation portable XRF analyzers are designed with a focus on convenience for process engineers. ProSpector 3, for example, delivers results in seconds: the typical measurement time is 3–5 seconds for express evaluation or about 15–60 seconds for higher precision. No complex sample preparation is required – a clean, flat coating surface is sufficient.

The instrument’s calibration already includes the main alloy types; if necessary, accuracy for a specific composition can be improved with minimal adjustment using just one reference sample. The analyzer interface intuitively displays the percentage composition of the main coating elements directly on the screen. These features make a portable XRF an instrument that a line engineer can use every day as an “electronic laboratory in a pocket”.

Use on real production objects.
The ProSpector 3 analyzer and similar devices are already successfully used at plants to control innovative coatings. On lines producing steels with Zn-Al-Mg coatings, for example, a portable XRF is used for incoming inspection of coils: at the edge of the coil or on a spot sample the percentages of Al and Mg are checked against their target values.

Instruments are also used by quality-control departments to audit subcontractors – when coated steel is purchased from third parties, incoming express analysis confirms the coating type (its chemical “fingerprint” makes it easy to distinguish, say, Galfan (5% Al) from conventional galvanizing or from Magnelis (Mg + Al)).

A key advantage of ProSpector 3 is its versatility: in addition to zinc coatings, it can analyze the steel itself (alloying elements in the base) and any other alloys, which is important in metallurgical production.

Thanks to the combination of mobility and laboratory-level accuracy, portable XRF analyzers have become an integral part of modern quality systems at metallurgical plants.

Examples of composition and applications of Zn-Al, Zn-Al-Mg and Zn-Mg coatings

The table below shows some common coating options, their approximate composition (mass fraction of elements) and main areas of application:

Conclusion

X-ray fluorescence analysis has proven itself as a reliable and convenient tool for ensuring high quality of zinc, aluminum-zinc and zinc-magnesium coatings in the steel industry. The ability of XRF to quickly and accurately measure Al, Mg, Si and other elements directly in production enables metallurgists to solve several key tasks at once: from maintaining the optimum bath composition during hot-dip galvanizing to checking every batch of rolled products for compliance with stringent standards.

Thanks to XRF, modern multi-component coatings with enhanced corrosion resistance (Zn-Al, Zn-Al-Mg, Zn-Mg) are produced consistently and predictably, which ultimately increases the durability of steel structures and customer confidence in the product. Next-generation instruments such as the ProSpector 3 handheld analyzer make control even more flexible – they bring the laboratory directly to the production line without sacrificing measurement accuracy.

For industry professionals it is obvious that compliance with regulatory requirements for coating composition (ASTM, EN and others) is impossible without regular analytical control, and in this context XRF has become the de-facto standard of analysis thanks to its combination of versatility, speed and cost-effectiveness. In summary, using XRF in galvanizing and steel rolling today is not just an option but a necessary element of the technology that ensures every micrometer of protective coating works efficiently, protecting the metal from corrosion throughout its service life.