Why Brush-Coated Titanium Anodes' Service Life Cannot Be Inferred Solely from XRF Thickness Measurement Results

In the acceptance process of titanium anode products, many customers adopt XRF (X-ray Fluorescence) to detect coating composition, which is a common and meaningful quality verification method. We understand and respect customers' emphasis on product quality, and also recognize the value of XRF in precious metal element identification, surface loading trend judgment and batch consistency control.

However, it needs to be specially explained that for titanium anodes prepared by the brush coating process, the XRF test results cannot be directly equivalent to the actual service life of the products, and even the compliance of anode service life cannot be inferred solely from XRF data. Simply equating the two is likely to lead to deviations in technical judgment, thereby affecting the objective evaluation of the real performance of the products.

This article aims to provide a clear explanation of this issue from the principle level.

The essence of XRF is to excite the surface elements of materials with X-rays and judge their types and relative contents according to the characteristic fluorescence emitted by different elements.For titanium anode products, XRF is mainly applicable to the following aspects:

First, detect whether the target elements are contained in the coating.For example, whether precious metal elements such as ruthenium, iridium, tantalum and platinum exist, and whether the element combination is basically consistent with the product type.

Second, make a trending judgment on the surface precious metal loading.It can help judge whether there are obvious high and low differences in the surface element content between samples, and can also be used for batch consistency management.

Third, assist in finding local surface abnormalities.Such as obvious thin or thick coating in some areas, or large fluctuations in the surface element distribution.

However, XRF itself has clear limitations. What it detects is element information, not service life information.It cannot directly tell us:
• Whether the combination between the coating and the substrate is firm;
• Whether the sintering state between each layer after multi-layer brush coating is sufficient;
• Whether there are microcracks, pores or local stress concentration inside the coating;
• At what rate the coating will deactivate under actual electrolysis working conditions;
• How working condition factors such as electrolyte composition, current density, temperature, start-stop frequency and polarity change will affect the final service life.

In other words, what XRF sees is the "composition", while the service life reflects the comprehensive result of "composition + structure + process + working conditions".

The essence of the XRF handheld analyzer measuring the mass of elements is to calculate the mass fraction of elements by combining the detected characteristic fluorescence intensity of elements with a formula, and then obtain the mass of elements per unit area by combining the detection area and coating thickness. The whole process requires no complex operation. The core principle is divided into three steps, and the formulas are explained centrally and simplified to avoid tedious symbols:

1. Excitation process: The X-ray tube inside the analyzer emits primary X-rays, which penetrate the surface of the anode coating, collide with the atoms of each element in the coating, and knock out inner electrons to form vacancies.

2. Fluorescence generation: When the outer electrons of atoms transition to the inner vacancies, exclusive characteristic fluorescent rays will be released (the fluorescence wavelength and energy of different elements are unique, for example, the characteristic fluorescence of titanium and iridium elements is significantly different).

3. Quantitative conversion: The detector captures the fluorescence and converts it into an electrical signal (i.e., fluorescence intensity). Combined with the calibration formula, the fluorescence intensity is converted into the mass fraction of elements, and then the mass is calculated-for the same element, the higher the content, the stronger the fluorescence intensity and the higher the calculated mass.

2.1 Core Formulas and Simplified Interpretation (Presented Centrally)

The core basis of XRF quantitative detection is the Lambert-Beer Law. The simplified formula adapted to the detection scenario is as follows (no complex derivation is needed, focusing on the key points related to element mass and detection deviation):

I=Io·ω·t·K

Simplified interpretation of the formula:

● I: Detected characteristic fluorescence intensity of the element (a directly measurable electrical signal value);

● Io: Primary X-ray intensity (a fixed parameter of the analyzer, calibrated in advance);

● ω: Mass fraction of the target element (the core quantity to be determined, the basis for converting the mass of the element);

● t: Coating thickness (a parameter detected synchronously by XRF, which can be combined with the mass fraction to calculate the mass of the element);

● K: Comprehensive calibration coefficient (a core standard variable and also the key leading to detection deviation, detailed below).

Supplementary explanation: In actual detection, the analyzer will automatically substitute this formula to convert the fluorescence intensity (I) into the mass fraction of elements (ω), and then directly display the mass of elements in combination with the detection area. The operation is convenient but greatly affected by the K value (comprehensive calibration coefficient).

2.2 Reasons for "Actually Qualified but XRF Test Unqualified" Caused by Standard Variable Differences (Explained in Combination with the Formula)

The comprehensive calibration coefficient K in the above formula is not a fixed value but composed of multiple standard variables. The setting or actual differences of these variables will lead to the deviation of the formula calculation results from the true values, thus resulting in the situation where "the product actually meets the standard but the XRF test shows unqualified". Combined with the detection scenario of brush-coated titanium anodes, the core standard variable differences and their impacts are as follows, illustrated with charts centrally:

1. Explanation of Core Standard Variables and Differences

The comprehensive calibration coefficient K consists of three categories of standard variables: "standard sample calibration parameters, matrix effect parameters and instrument hardware parameters". The difference of each category of variables will affect the final test results, as detailed below:

2. Popular Explanation Combined with Actual Scenarios

Taking our brush-coated titanium anodes as an example, assuming that the actual iridium mass in the product coating fully meets your standard (i.e., the true ω value is up to standard), the XRF test may show "insufficient iridium mass and unqualified" due to the following standard variable differences:

(1) Mismatched standard samples: When the analyzer leaves the factory, the K value is calibrated with the "standard sample of titanium anode with spray coating process", while our products adopt the "brush coating process". The porosity and bonding state of the brush-coated coating are different from those of the spray-coated standard sample, leading to the inconsistency between the K value setting and the actual situation. After substituting into the formula, the calculated ω value is low, and the product is mistakenly judged as unqualified.

(2) Matrix effect impact: Slight oxidation on the surface of the titanium matrix (which does not actually affect the anode performance) will absorb part of the characteristic fluorescence of iridium elements, resulting in a low measured value of I (fluorescence intensity). According to the formula I = Io·ω·t·K, with Io, t and K unchanged, the ω value will be mistakenly judged as low, i.e., the iridium mass is insufficient.

(3) Hardware operation differences: A deviation of 1-2mm in the distance between the probe and the coating or an inclined detection angle during handheld XRF detection will lead to the measurement deviation of the I value, and then the wrong element mass is converted through the formula, resulting in the situation of "actually qualified but unqualified in detection".

In addition, XRF detection is a qualitative and semi-quantitative detection by nature. Limited by power and resolution, it cannot achieve high-precision quantification like large laboratory equipment. The deviation of its test results is an objective existence, which is also one of the important reasons why it cannot be used as the sole basis for anode service life judgment.

The coating of brush-coated titanium anodes is not simply a matter of "laying a certain metal on the surface". Its formation process usually includes: pre-treatment, liquid preparation, fractional brush coating, fractional drying, fractional thermal decomposition/sintering, and the final multi-layer composite film formation. What ultimately works is not the "amount of elements detected on the surface at a certain moment", but the stability of the entire coating system under actual working conditions.

3.1 Core Misunderstanding: XRF Thickness Measurement ≠ The Whole Basis for Service Life Judgment

As a non-destructive testing method, XRF (X-ray Fluorescence Spectroscopy) qualitatively and quantitatively analyzes the coating element composition and thickness by exciting the characteristic fluorescent rays generated by sample atoms. It has the advantages of speed and non-destructiveness, and is suitable for batch screening, but it has three key limitations that make it impossible to directly deduce the service life:

3.1.1 Inherent Deviation in Thickness Measurement Logic

The coating thickness and service life are not simply "positively correlated", let alone "the thicker the more durable". The brush-coated titanium anode adopts the "brush coating - thermal decomposition" process, and the coating thickness is usually controlled at 5-20μm, with a clear reasonable range in the industry:

● Too thin (<5μm): Insufficient active components, easy to be consumed quickly, and shortened service life;

● Too thick (>25μm): Mismatched thermal expansion coefficients between the coating and the titanium matrix (titanium matrix α≈8.6×10⁻⁶/℃, iridium coating α≈6.5×10⁻⁶/℃), internal stress is generated after sintering, and microcracks are easy to appear after cooling, which instead accelerates peeling and reduces service life.

Our brush coating process strictly follows the reasonable industry range, and achieves the balance between thickness uniformity and bonding force by controlling the number of brush coating passes (8-15 passes) and the solvent proportion (n-butanol 20%-40%), avoiding the risk of "failure due to excessive thickness".

3.1.2 Core Performance Dimensions Not Covered by Thickness Measurement

The service life of titanium anodes is jointly determined by multiple factors such as coating composition, bonding strength, porosity and electrocatalytic activity, and XRF cannot detect these key indicators at all:

3.1.3 Interference from the Complexity of Actual Working Conditions

XRF thickness measurement results are easily affected by the surface state. For example, oil stains, oxide layers and scaling on the coating surface will increase the measurement error from 5% to 15%, failing to reflect the real coating state. The real failure of titanium anodes often results from electrochemical dissolution, gas scouring and local corrosion, which will gradually consume active components and have no direct correlation with the initial thickness measurement data.

3.2 Service Life Depends on the "Effective Coating System", Not Just the "Surface Element Values"

For the same brush coating process, even if the signals of certain elements measured on the surface of two anodes are close, it does not mean that their deactivation rates during energized operation must be the same.

The reason is that the service life is determined by the comprehensive performance of the coating system in long-term operation, including:

● Whether the coating is uniform and continuous;

● Whether the precious metal oxide forms a stable and effective active layer;

● Whether there is a good bonding state between the coating and the titanium matrix;

● Whether a micro-structure suitable for the target working conditions is formed after repeated multi-layer heat treatment.

These key factors are not the strengths of XRF.

3.3 XRF is Closer to "Surface Element Identification", but Service Life is a "Dynamic Service Result"

The service life of titanium anodes is not a static concept, but a process of gradual consumption and deactivation in an electrochemical environment.

What customers really care about by "service life" is essentially: how long the anode can maintain an acceptable working state under the specified working conditions.

This question can only be answered under actual or accelerated electrochemical conditions.

Because the anode failure process may involve:

● Gradual consumption of active components;

● Changes in the surface and internal structure of the coating;

● Preferential attenuation in local areas;

● Decrease in the protection capacity of the substrate;

● Increase in polarization after long-term operation.

These all belong to "service behavior", not simply "the presence or absence of components".

3.4 Brush-Coated Products Have Hierarchy and Local Differences, Making Single-Point XRF Even Harder to Represent the Overall Service Life

The brush-coated coating is established step by step through multiple coating and heat treatment processes.

Therefore, its final state is often characterized by hierarchy, processability and certain regional differences. If customers adopt the XRF test results of limited points and then directly convert the results into service life, two problems are likely to occur:

First, the detection points may not represent the whole.

The surface signal of local points may not fully reflect the state of the effective working layer of the entire anode.

Second, XRF results cannot be automatically converted into a service life model.

Even if there is a general trend in experience that "the higher the loading, the more conducive it is to the service life", it does not mean that the one-to-one service life conversion can be done without considering the specific process, specific formula and specific working conditions.

In other words, XRF can help judge whether the "surface composition of the product is basically reasonable", but cannot independently complete the "certification of service life conclusions".

We understand that customers hope to use a fast method to judge the product, but drawing the direct conclusion that "the service life fails to meet the standard" based on this is still technically insufficient.

The main reasons are threefold.

First, the inconsistency between the detection object and the evaluation object

XRF detects element composition and surface signals;

Service life evaluation focuses on electrochemical service capacity and stable time.

The two are related, but they are not the same indicator, let alone directly replaceable indicators.

Second, the lack of working condition boundaries makes the service life conclusion lack a foundation for establishment

The service life of any anode must correspond to clear service conditions, such as:

Current density;

Electrolyte system;

Temperature range;

pH conditions;

Medium conditions such as chloride ions and fluoride ions;

The presence of frequent start-stop or reverse polarity operations.

Judging whether the service life meets the standard solely by the element signals measured by XRF without specific working conditions itself lacks the establishment conditions.

Because the service life performance of the same anode may vary significantly under different working conditions.

Third, ignoring that the core impact of the brush coating process on service life comes from the "quality of process implementation"

For brush-coated titanium anodes, the formula is only the foundation, and what truly converts the formula into service life performance is the quality of process control, including:

● Whether the substrate pre-treatment is sufficient;

● Whether the coating liquid preparation is stable;

● Whether each brush coating is uniform;

● Whether each drying and thermal decomposition reaches the desired state;

● Whether the final film forms a stable, continuous and well-adhered active system.

Therefore, the service life judgment must be based on the comprehensive evaluation of composition, process, structure and working conditions, and cannot be simplified to a single XRF conclusion.

If the goal is to truly guarantee the service life of the anode, rather than just making a quick judgment on the surface composition, a more reasonable method should be established around "performance verification" and "risk sharing".

We believe it should include at least the following two aspects.

5.1 Verify Service Life Through Enhanced Service Life Testing Instead of Replacing It with XRF

The authoritative method for evaluating the service life of titanium anodes in the industry is enhanced service life testing, which is also the specified acceptance basis in national standards such as Titanium Anodes for Cathodic Protection (YS/T 828-2022). The core logic is "accelerated working condition simulation → quantitative failure threshold → conversion of actual service life".

The so-called enhanced service life testing is essentially to conduct continuous assessment of the anode under stricter or more easily accelerated failure conditions than the actual working conditions, so as to observe the change trend of its stability more quickly. Its purpose is not simply to "get a number", but to simulate the attenuation mechanism that the anode may experience in long-term operation as much as possible.

Why is this method more reasonable?

1.It evaluates the "working performance" rather than the "surface composition appearance"

Service life is inherently a performance in work, so service life verification should be carried out under the conditions of energization, medium, temperature and so on. Although the enhanced service life test is not a simple copy of the actual on-site service life, at least its evaluation logic is consistent with the "service life" itself, that is, to see whether the anode is stable during continuous operation, when obvious attenuation occurs, and whether the attenuation process meets expectations.

2.It can truly reflect the impact of process quality

As mentioned earlier, the service life of brush-coated titanium anodes largely depends on the quality of process implementation.

The enhanced service life test can exactly "stimulate" these factors:

● Coatings with poor bonding will expose problems earlier;

● Coatings with unstable structure will show performance attenuation earlier;

● Differences caused by process fluctuations are also easier to be identified in the test. 

This is closer to the real capacity of the product than just looking at the XRF surface element data.

3.It is conducive to the formation of a mutually recognized evaluation standard between suppliers and customers

If customers are worried about service life risks, the most effective method is not to make unilateral inferences only with XRF results, but for both parties to agree in advance on:

● Sample type;

● Test medium;

●Current conditions;

●Failure judgment method;

● Comparative sample or historical reference method.

The test conclusion formed in this way is more convincing and more conducive to reaching an agreement between the two parties.

5.2 Actually Guarantee the Service Life Risk Through a Quality Assurance Deposit Mechanism, and Use It for Anode Re-processing When Necessary

In addition to test verification, another way that can better reflect the sense of responsibility is to establish a quality assurance deposit mechanism.

The core of this idea is not to argue about "how to calculate on paper", but to focus on "how to solve the problem if the actual operation is inconsistent with the agreement".

1.The significance of the quality assurance deposit mechanism is to implement the quality commitment to the execution level

For customers, what they really care about is not a single test value, but whether the product can work stably in the project.

Through the arrangement of the quality assurance deposit, the two parties can agree that a part of the payment is used as a quality assurance measure, which will be released or disposed of according to the agreed conditions after the product is put into actual application.

The value of this method is that:

It converts the "verbal commitment" into an "executable arrangement", allowing customers to see the supplier's willingness to bear the responsibility for service life.

2.The quality assurance deposit can be used as a guarantee source for subsequent re-processing

For brush-coated titanium anodes, if some anodes are found to need reprocessing in the subsequent actual operation, a part of the quality assurance deposit can be directly used for:

● Factory return inspection;

● Surface treatment;

● Re-brush coating;

● Re-heat treatment;

● Performance recovery processing.

This is more practically meaningful than simply arguing about "whether the theoretical service life is sufficient" based on XRF.

Because what the customer's project needs is sustainable operation, not staying at the level of test explanation.

3.This method is more in line with the logic of engineering cooperation

For engineering products, especially electrochemical material products, the entire quality result cannot be defined by a single static parameter in many cases.

A more mature cooperation method should be:

● Conduct necessary composition and process quality control before delivery;

● Conduct enhanced service life verification before delivery;

● Undertake actual risks through the quality assurance deposit and reprocessing mechanism after delivery.

In this way, both suppliers and customers focus on the "project results" rather than being limited by a single test conclusion.

Based on the above analysis, we suggest that the judgment of the service life of brush-coated titanium anode products should adhere to the following principles:

First, XRF can be used as a quality control tool, but not a single service life adjudication tool.

It is suitable for verifying the presence of surface elements, the rationality of the surface loading trend and batch consistency, but should not be directly equivalent to the service life conclusion.

Second, the service life judgment should be based on testing and working conditions.

Only by combining clear application conditions and verifying through enhanced service life testing or actual operation can a more objective judgment be made on whether the service life meets the standard.

Third, quality assurance should not only stay at the testing level, but also be reflected in the responsibility arrangement.

Through the quality assurance deposit and reprocessing mechanism when necessary, customers can obtain more practical and executable guarantees.

For electrochemical functional materials such as brush-coated titanium anodes, composition detection is important, but composition is not equal to service life; XRF is valuable, but XRF cannot replace service life evaluation.

Directly using XRF results to infer service life and judging that the product fails to meet the service life standard based on this is technically incomplete and may lead to misjudgment of product performance.

A truly responsible approach that is more in line with engineering logic should be:

● Use XRF for composition and consistency verification;

● Use enhanced service life testing to verify the service life trend;

● Use the quality assurance deposit and reprocessing mechanism to bear the actual quality responsibility.

We are willing to work with customers to establish a more reasonable and transparent quality evaluation system in this way. Because for titanium anode products, what really matters is not a single surface test number itself, but whether the product can complete its work stably, reliably and sustainably in actual application.

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