—An Explanation of Coating Composition Testing, Service Life Assessment, and Quality Assurance Methods
During the acceptance process for titanium anode products, many customers use XRF (X-ray fluorescence) to analyze coating composition, which is a common and meaningful method of quality verification. We understand and respect our customers’ emphasis on product quality, and we acknowledge the value of XRF in identifying precious metal elements, assessing trends in surface loading, and ensuring batch consistency.
However, it is important to note that for titanium anodes produced via brush-coating processes, XRF test results only reflect the elemental content within a specific thickness range of the product. These results cannot be directly equated with the actual precious metal loading of the product, nor can they be used to estimate actual service life, and certainly cannot be used alone to determine whether the anode meets quality standards. Simply equating the two can easily lead to technical misjudgments, thereby increasing the cost of anode products and indirectly affecting the objective evaluation of their actual performance.
I. What XRF Can and Cannot Detect
XRF stands for X-ray Fluorescence Spectroscopy. It utilizes X-rays to excite elements on the surface of a material, causing them to emit characteristic fluorescent X-rays for elemental qualitative and quantitative analysis.
For titanium anode products, XRF is primarily applicable in the following areas:
First, qualitative analysis of coating elements in anode products.
This is a process to determine the “presence or absence” of specific elements. Elements such as ruthenium, iridium, tantalum, and platinum—which may be present in precious metal coatings—can be identified.
Second, trend analysis of the loading ratio of precious metals on the surface layer.
XRF test results provide information on the types of precious metals present in the coating area and their corresponding proportions. Using this data, the approximate proportion of precious metals in the anode coating can be estimated. For anodes with high precious metal content, this data can be used for a preliminary assessment of anode quality. It allows for a rough determination of whether there are significant differences in the content or proportion of coating elements based on precious metal ratios, and can also be used for consistency management across batches.
However, XRF itself has clear limitations. It detects elemental information, not lifespan information.
It cannot directly tell us:
♦ Whether the coating is firmly bonded to the substrate;
♦ Whether the sintering between layers is sufficient after multi-layer coating;
♦ Whether there are microcracks, pores, or localized stress concentrations within the coating;
♦ At what rate the coating will degrade under actual electrolytic operating conditions;
♦ How operating factors such as electrolyte composition, current density, temperature, start-stop frequency, and polarity changes affect the final service life.
In other words, XRF measures “composition,” while service life reflects the combined result of “composition + structure + process + operating conditions.”
II. The Principle and Core Formula for Measuring Elemental Mass in Grams Using an XRF Gun
The process by which an XRF gun measures elemental content essentially involves detecting the intensity of characteristic fluorescence, converting it into mass fraction using a formula, and then combining this with the detection area and coating thickness to ultimately determine the mass of the element per unit area. The entire process requires no complex operations. The core principle consists of three steps, with the formula explained concisely and simplified to avoid cumbersome notation:
1. Excitation Process: The X-ray tube inside the probe emits primary X-rays, which directly irradiate atomic elements within a specific thickness range of the coating (determined by the X-ray energy). These X-rays bombard inner-shell electrons, creating vacancies in the electronic structure;
2. Fluorescence Generation: When outer-shell electrons of an atom transition to inner-shell vacancies, they emit characteristic fluorescent radiation (the wavelength and energy of fluorescence are unique to each element; for example, the characteristic fluorescence of pure titanium and iridium differs significantly);
3. Quantitative Conversion: After the detector captures the fluorescence, it is converted into an electrical signal (i.e., fluorescence intensity). Using calibration formulas, the fluorescence intensity is converted into the mass fraction of the element, and the mass in grams is then calculated—for the same element, the higher the concentration, the stronger the fluorescence intensity, and the higher the calculated mass in grams.
2.1 Core Formulas and Simplified Interpretation (Consolidated Presentation)
The core principle of XRF quantitative analysis is the Lambert-Beer Law. After adapting it to the detection scenario, the simplified formula is as follows (no complex derivations are required; the focus is on key points related to elemental mass and detection error):
Simplified Interpretation of the Formula:
I=Io·ω·t·K
I: Detected characteristic fluorescence intensity of the element (directly measurable electrical signal value);
I₀: Primary X-ray intensity (a fixed parameter of the detector gun, calibrated in advance);
ω: Mass fraction of the target element (the core variable to be determined, serving as the basis for calculating elemental mass);
t: Irradiation depth (a parameter measured simultaneously by the XRF; combined with the mass fraction, it enables the calculation of elemental mass);
K: Comprehensive calibration coefficient (a core standard variable and a key factor causing measurement deviation; detailed explanation follows)
Additional Note: In actual testing, the XRF analyzer automatically applies this formula to convert the fluorescence intensity (I) into the elemental mass fraction (ω). Combined with the measurement area, it then directly displays the elemental mass in grams. While this process is convenient, it is highly susceptible to variations in the K-value (comprehensive calibration coefficient).
2.2 Reasons for “Products Meeting Standards but Failing XRF Testing” Due to Variations in Standard Variables (with Formula Explanation)
In the formula above, the comprehensive calibration coefficient K is not a fixed value but is composed of multiple standard variables. Differences in the settings or actual values of these variables can cause the calculated result to deviate from the true value, leading to situations where “the product actually meets the standard, but the XRF test indicates failure.” Taking the testing scenario of brushed titanium anodes as an example, the core standard variable differences and their impacts are as follows, illustrated in the table below:
1. Core Standard Variables and Explanation of Differences
The comprehensive calibration coefficient K is composed of three major categories of standard variables: “standard sample calibration parameters, matrix effect parameters, and instrument hardware parameters.” Differences in each category of variables will affect the final test results, as detailed below:
| Standard Variable Type | Specific Variable Content | Variable Difference Performance | Impact on Test Results (Formula Explanation) |
|---|---|---|---|
| Standard Sample Calibration Parameters | Composition, coating thickness, process of calibration standard samples | Standard samples for factory calibration differ from actual coating process and composition ratio of brush‑coated anodes (e.g., standard samples use spraying process, our product uses brush coating) | Standard sample differences cause K value setting deviation. Even if actual ω (elemental mass fraction) meets the standard, calculated I (fluorescence intensity) is low, misjudged as “insufficient elemental weight, unqualified” |
| Substrate Effect Parameters | Absorption and enhancement coefficients of titanium substrate for characteristic fluorescence | Different oxidation degree and impurity content of titanium substrate lead to different absorption/enhancement effects on coating element fluorescence (i.e., substrate effect difference) | Substrate effect changes absorption/enhancement coefficients in K value, making measured I deviate from true value: excessive absorption lowers I (misjudged as insufficient weight); excessive enhancement raises I (misjudged as excessive) |
| Instrument Hardware Parameters | X‑ray tube power, detector resolution, test angle | Different brands/models of XRF testers have different hardware settings (handheld XRF power: 5–50W, laboratory equipment: hundreds of watts); probe distance and angle vary in operation | Hardware differences affect measurement accuracy of I₀ (primary X‑ray intensity) and I (fluorescence intensity), causing deviation in calculated ω (mass fraction) and misjudging product qualification |
| Other Auxiliary Variables | Test ambient temperature, coating surface condition | Extreme on‑site temperature, or oil stains, oxide layers, scaling on coating surface | Temperature affects detector sensitivity; surface impurities absorb fluorescence, causing I deviation and misjudging elemental weight as unqualified |
2.3 Practical Examples
Taking our brush-applied titanium anodes as an example, suppose the actual iridium content in the product’s coating fully meets the order specifications (i.e., the true ω value is within specifications). However, due to the following standard variable discrepancies, XRF testing may indicate “insufficient iridium content, non-conforming”:
(1) Mismatch with the reference sample: When the XRF gun was shipped from the factory, it was calibrated using a “spray-coated titanium anode reference sample” to set the K-value. However, our product is produced using a “brush-coated process.” The porosity and bonding state of the brush-coated layer differ from those of the spray-coated reference sample, causing the K-value setting to deviate from reality. When substituted into the formula, the calculated ω-value is too low, leading to a false non-conformance judgment;
(2) Substrate effect: A brush-coated anode combines a surface-treated titanium substrate with precious metal oxides to form the final product. The principle involves the formation of a TiO₂ layer of a certain thickness on the substrate surface following surface treatment. Due to the treatment process, the substrate surface develops a uniformly distributed microporous structure composed of TiO₂. Compounds of iridium and ruthenium/tantalum in the precious metal coating settle into these microporous structures. During sintering and multiple stacking processes, the precious metals and TiO₂ form a tightly bonded co-site rutile structure. This unique structure ensures the bonding strength between the precious metal coating and the substrate, while also enabling outer-shell electron conjugation among Ir, Ru/Ta, Ti, and O. This results in the absorption of part of the characteristic fluorescence of iridium, leading to a lower measured value of I (fluorescence intensity). According to the formula I = I₀·ω·t·K, when I₀, t, and K remain constant, this may lead to an erroneous determination of a lower ω value, i.e., an underestimation of the iridium content;
(3) Differences in hardware operation: When performing handheld XRF detection, a deviation of 1–2 mm in the distance between the probe and the coating, or an angled detection position, can cause measurement errors in the I value, which in turn affects the calculated result via the formula.
III. Why XRF Alone Is Insufficient for Estimating the Service Life of Brush-Coated Titanium Anodes
Coating a brush-applied titanium anode is not simply a matter of “spreading” a metal onto the surface. The process typically involves: pretreatment, solution preparation, sequential brush application, sequential drying, sequential thermal decomposition/sintering, and the final formation of a multi-layer composite film. What ultimately matters is not the “elemental content measured on the surface” at any given moment, but the stability of the entire coating system under actual operating conditions.
3.1 Key Misconception: XRF Thickness Measurement ≠ Sole Basis for Life Assessment
As a non-destructive testing method, XRF (X-ray fluorescence spectroscopy) offers the advantages of speed and non-destructiveness, making it suitable for batch screening. However, it has three key limitations that prevent it from directly determining service life:
3.1.1. Inherent Bias in Thickness Measurement Logic
Coating thickness and service life are not simply “positively correlated,” nor does “thicker” necessarily mean “more durable.” Brush-coated titanium anodes use a “brush coating – thermal decomposition” process, with coating thickness typically controlled between 5–20 μm. The industry has established a clear, reasonable range:
● Too thin (<5 μm): Insufficient active components, prone to rapid depletion, resulting in shortened service life;
● Too thick (>25 μm): The thermal expansion coefficients of the coating and the titanium substrate do not match (titanium substrate α ≈ 8.6 × 10⁻⁶/°C, iridium coating α ≈ 6.5 × 10⁻⁶/°C). This causes internal stress after sintering, leading to microcracks upon cooling, which in turn accelerates peeling and reduces service life. Our brush-coating process strictly adheres to industry-standard parameters. By controlling the number of brush passes (8–15) and the solvent ratio (n-butanol 20%–40%), we achieve a balance between thickness uniformity and adhesion, thereby avoiding the risk of “failure due to excessive thickness.”
XRF is relatively accurate for testing the thickness of metal coatings, but its accuracy for testing oxide coatings is debatable.
Due to the limited excitation intensity of X-rays in handheld devices, the penetration depth of the X-rays and the escape depth of characteristic X-rays are restricted, resulting in limitations on the detectable thickness range.
3.1.2. Core Performance Dimensions Not Covered by Thickness Measurement
The lifespan of a titanium anode is determined by multiple factors, including coating composition, adhesion strength, porosity, and electrocatalytic activity. XRF is completely incapable of detecting these key indicators:
| Key Indicator | XRF Detection Capability | Impact on Life |
|---|---|---|
| Coating composition (e.g., Ru/Ir ratio) | Detects content but cannot judge active component effectiveness | Ru/Ir oxides are core for chlorine/oxygen evolution; unbalanced ratio reduces current efficiency and accelerates failure |
| Bonding strength | Undetectable | Coating peels easily when adhesion <5MPa, short life even with qualified thickness |
| Porosity | Undetectable | High porosity accelerates electrolyte penetration, forms non‑conductive TiO₂ passivation layer, causes performance decay |
| Electrocatalytic activity | Undetectable | Directly determines energy consumption and stability, core for long‑term operation |
3.1.3. Interference from the Complexity of Actual Operating Conditions
XRF thickness measurements are easily influenced by surface conditions, such as oil contamination, oxide layers, or scaling on the coating surface, which can increase measurement errors from 5% to 15%, failing to reflect the true state of the coating. However, the actual failure of titanium anodes often stems from electrochemical dissolution, gas erosion, and localized corrosion—processes that gradually deplete active components and have no direct correlation with initial thickness data.
3.2 XRF Is Closer to “Elemental Identification,” but Service Life Is a “Dynamic Operational Result”
Even for the same brush-coating process, if the measured signals of certain elements on the surfaces of two anodes are similar, this does not necessarily mean their deactivation rates during electrical operation will be the same.
The reason is that service life is determined by the comprehensive performance of the coating system during long-term operation, including:
• Whether the coating is uniform and continuous;
• Whether precious metal oxides have formed a stable and effective active layer;
• Whether there is good adhesion between the coating and the titanium substrate;
• Whether a microstructure suitable for the target operating conditions is formed after repeated multi-layer heat treatment.
XRF cannot identify these key factors.
3.3 XRF is closer to “surface element identification,” but service life is a “dynamic operational result”
The service life of a titanium anode is not a static concept, but rather a process of gradual consumption and deactivation in an electrochemical environment.
The “lifespan” that customers truly care about is, in essence: how long the anode can maintain an acceptable operational state under specified operating conditions.
This question can only be answered under actual or accelerated electrochemical conditions.
This is because the anode failure process may involve:
• Gradual depletion of active components;
• Changes in the coating’s surface and internal structure;
• Preferential degradation in localized areas;
• Decline in the substrate’s protective capability;
• Increased polarization after long-term operation.
These all fall under “service behavior” rather than simply “
3.4 Brush-applied coatings exhibit layered structures and local variations, making it even more difficult for single-point XRF measurements to represent overall service life
Brush-applied coatings are built up gradually through multiple coating applications and heat treatments.
As a result, their final state often exhibits layered structures, process-related characteristics, and certain regional variations. If customers use XRF test results from a limited number of points and directly convert those results into service life estimates, two problems are likely to arise:
First, the selection of test points is highly random and cannot represent the overall result.
Surface signals from localized points may not fully represent the condition of the effective working layer across the entire anode.
Second, XRF results cannot be automatically converted into a service life model.
Even if there is a general empirical trend that “higher load capacity typically leads to longer service life,” this does not mean that a direct, one-to-one conversion to service life can be made without considering the specific process, formulation, and operating conditions.
In other words, XRF can help determine whether “the product composition is fundamentally sound,” but it cannot independently validate “conclusions regarding service life.”
IV. Why the Conclusion That “XRF Results Indicate the Product Fails to Meet Service Life Standards” Is Not Rigorous
We understand that customers wish to evaluate products quickly, but drawing the conclusion that “the service life does not meet standards” based solely on this is still insufficient from a technical standpoint.
There are three main reasons for this.
First, the subject of testing and the subject of evaluation are not the same.
XRF measures elemental composition and signal intensity,
while service life evaluation assesses electrochemical performance and stability over time.
While the two are related, they are not the same metric, nor are they directly interchangeable.
Second, the absence of operating conditions renders the service life conclusion unfounded
The service life of any anode must correspond to specific operating conditions, such as:
♦ Current density;
♦ Electrolyte system;
♦ Temperature range;
♦ pH conditions;
♦ Presence of chloride ions, fluoride ions, and other media;
♦ Whether frequent start-stop cycles or reverse polarity operations occur.
If one attempts to determine whether the service life meets standards based solely on elemental signals measured by XRF without considering specific operating conditions, the conclusion lacks a valid basis.
This is because the service life performance of the same anode may vary significantly under different operating conditions.
Third, the core impact of the brush-coating process on service life stems from “process execution quality.”
For brush-coated titanium anodes, the formulation is merely the foundation; what truly translates the formulation into service life performance is the quality of process control, including:
♦ Whether substrate pretreatment is thorough;
♦ Whether the coating solution preparation is stable;
♦ Whether each brush coating application is uniform;
♦ Whether each drying and thermal decomposition step achieves the required state;
♦ Whether the final coating forms a stable, continuous, and well-adhered active system.
Therefore, service life assessment must be based on a comprehensive evaluation of composition, process, structure, and operating conditions, and cannot be simplified to a single XRF conclusion.
V. What Constitutes a More Reasonable Approach to Service Life Assurance
If the goal is to genuinely ensure the service life of the anode—rather than merely conducting a quick assessment of surface composition—then a more reasonable approach should be based on “performance verification” and “risk sharing.”
We believe this should include at least the following two aspects.
5.1 Verify service life through enhanced service life testing, rather than substituting XRF for service life testing
The authoritative method for evaluating the service life of titanium anodes within the industry is accelerated life testing. This is also the acceptance criterion explicitly stipulated in national standards such as *Titanium Anodes for Cathodic Protection* (YS/T 828-2022). The core logic is “simulation of accelerated operating conditions → quantification of failure thresholds → conversion to actual service life.”
Essentially, accelerated life testing involves subjecting the anode to continuous evaluation under conditions that are more severe than actual operating conditions or that more readily accelerate failure, thereby allowing for faster observation of trends in stability changes. The purpose is not merely to “produce a number,” but to simulate as closely as possible the degradation mechanisms that may occur during the anode’s long-term operation.
Why is this approach more reasonable?
1. It evaluates “operational performance,” not “surface composition appearances”
Lifespan is inherently a measure of performance during operation; therefore, lifespan verification should be conducted under conditions such as power supply, electrolyte, and temperature. Although accelerated life testing is not a simple replication of actual field lifespan, its evaluation logic aligns with the concept of “lifespan” itself—namely, assessing whether the anode remains stable during continuous operation, when significant degradation occurs, and whether the degradation process meets expectations.
2. It truly reflects the impact of process quality
As mentioned earlier, the lifespan of brush-coated titanium anodes depends largely on the quality of the manufacturing process.
Stress life testing is precisely designed to “trigger” these factors:
• Poorly bonded coatings will reveal issues earlier;
• Structurally unstable coatings will show performance degradation sooner;
• Variations caused by process fluctuations are also more easily identified during testing.
This provides a more accurate picture of the product’s true capabilities than relying solely on XRF surface element data.
3. It helps both suppliers and customers establish mutually recognized evaluation standards
If a customer is concerned about lifespan risks, the most effective approach is not to make unilateral inferences based solely on XRF results, but rather for both parties to agree in advance on:
• Sample types;
• Test medium;
• Current conditions;
• Failure determination criteria;
• Comparison samples or historical reference methods.
Test conclusions formed in this manner are more persuasive and facilitate mutual agreement between both parties.
5.2 Providing practical safeguards against service life risks through a performance bond mechanism, and utilizing it for anode reprocessing when necessary
In addition to testing and verification, another approach that better demonstrates a responsible attitude is establishing a performance bond mechanism.
The core of this approach is not to debate “how calculations are performed on paper,” but to focus on “how to resolve issues if actual operation deviates from the agreed terms.”
1. The significance of the retention deposit mechanism lies in translating quality commitments into concrete action
For customers, the true concern is not a single test value, but whether the product can operate stably within the project.
Through a retention fund arrangement, both parties can agree to set aside a portion of the payment as a quality assurance measure, to be released or disposed of according to agreed-upon conditions after the product enters actual use.
The value of this approach lies in:
It transforms “verbal commitments” into “enforceable arrangements,” allowing the customer to see the supplier’s willingness to assume responsibility for product lifespan.
2. The retention deposit can serve as a source of funding for subsequent reprocessing
For brush-coated titanium anodes, if it is discovered during actual operation that some anodes require reprocessing, a portion of the retention deposit can be directly used for:
♦ Return-to-factory testing;
♦ Surface treatment;
♦ Re-coating;
♦ Re-heat treatment;
♦ Performance restoration processing.
This approach holds greater practical significance than simply debating whether “theoretical lifespan is sufficient” based solely on XRF data.
This is because the client’s project requires sustainable operation, not merely analysis at the testing and interpretation level.
3. This approach better aligns with the logic of engineering collaboration
Engineering products, particularly electrochemical materials, often cannot be fully defined by a single static parameter.
A more mature collaborative approach should involve:
♦ Conducting necessary composition and process quality control prior to shipment;
♦ Conducting enhanced lifespan validation prior to delivery;
♦ Assuming actual risk post-delivery through retention of warranty deposits and reprocessing mechanisms.
In this way, both suppliers and customers focus their attention on “project outcomes” rather than being constrained by the conclusions of any single test.
VI. We Recommend Returning to Sound Logic in Assessing the Service Life of Titanium Anodes
Based on the above analysis, we recommend that the following principles be adhered to when assessing the service life of brush-coated titanium anode products:
First, XRF can serve as a quality control tool, but it should not be used solely as a tool for determining service life.
It is suitable for verifying the presence of surface elements, assessing whether surface loading trends are reasonable, and ensuring batch consistency; however, it should not be directly equated with conclusions regarding service life.
Second, service life assessments should be based on testing and actual operating conditions.
Only by combining specific application conditions with accelerated life testing or verification through actual operation can a more objective judgment be made regarding whether the service life meets standards.
Third, quality assurance should not be limited to testing; it must also be reflected in accountability arrangements.
Through quality assurance deposits and, when necessary, a rework mechanism, customers can obtain more practical and enforceable guarantees.
VII. Conclusion
For electrochemical functional materials such as brush-coated titanium anodes, composition testing is important, but composition does not equate to service life; XRF is valuable, but it cannot replace service life evaluation.
If XRF results are used directly to infer service life and to conclude that a product fails to meet service life standards, this approach is technically incomplete and may lead to misjudgments of product performance.
A truly responsible approach that better aligns with engineering logic should be:
• Use XRF as a reference for composition and consistency;
• Verify service life trends through accelerated life testing;
• Assume actual quality responsibility through quality assurance deposits and reprocessing mechanisms.
We are willing to work with our customers to establish a more reasonable and transparent quality evaluation system in this manner. Because for titanium anode products, what truly matters is not a single surface testing figure in itself, but whether the product can perform its function stably, reliably, and sustainably in actual applications.
We fully understand that the cost of precious metals can be a real pain point for buyers~
If you want to see exactly how much precious metal you’re purchasing, we recommend XRF as a reference;
If you want to know how long it will last, we strongly recommend a comprehensive evaluation;
If you want to purchase reliable, long-lasting titanium anodes, we recommend Ehisen !
