ASTM D6903 Organic Biocide Release Rate From Antifouling Coatings in Substitute Ocean Water

Get Certified ASTM D6903 Testing for Reliable Antifouling Biocide Release Data

An antifouling coating must release its biocide at a controlled rate to keep surfaces free of marine growth. If it leaks out too rapidly, the protection might wear off before the next maintenance visit. ASTM D6903 provides a controlled laboratory procedure for comparing how antifouling coatings release organic biocides when immersed in substitute ocean water. The results can assist formulation teams in adjusting the amount of biocide used based on the measured release data. As a result, the testing aids in material selection, formulation development, quality control, and the assessment of coating performance under specified laboratory conditions.

The scope of ASTM D6903 includes

• Material type: Antifouling coatings that contain organic biocide

• Property measured: Rate of biocide release into substitute ocean water

• Test conditions: Controlled pH, temperature, salinity, and hydrodynamics, run entirely in the laboratory.

• Duration: The effect lasts long enough for results to develop over at least 45 days of exposure.

• Biocides covered: DCOIT (4, 5-dichloro-2-n-octylisothiazolin-3-one), CuPT (copper pyrithione), ZPT (zinc pyrithione) and CDMTD (N-cyclopropyl-N′-(1,1-dimethylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine)

• Method: Analysts take water samples at regularly determined intervals and then analyze them for the presence of the leached biocide.

• Analysis: The appendices provide HPLC methods for each biocide, and analysts may use a different method as long as it meets the standard’s acceptability criteria.

• Detection level: The detection level is the amount of biocide measured in the water at the low parts-per-billion level.

• Copper option: If the coating includes a copper-based biocide, analysts should measure the amount of copper released at the same time using Test Method D6442.

• Result: Release rate in µg cm⁻² d⁻¹, with SI units as the standard

• Quantification range: The maximum values are 500 µg cm⁻² d⁻¹ per biocide, while the minimum values are 4.5 for DCOIT, 0.36 for CuPT and ZPT, and 2.7 for CDMTD; when the procedures in the appendix are followed, these minimum values are reduced to 3.8, 0.16, 0.2 and 2.2 respectively. When using lower rates, an analytical method with a lower limit of quantitation is required.

• Light protection: When testing a highly light-sensitive biocide, analysts protect the apparatus and the samples from visible light.

• Limitations: The findings do not account for environmental release rates and are therefore unsuitable for risk assessments, loading estimates, or regulatory release limits.

• Applications: Formulation comparison, material selection, and quality control

What are the uses of ASTM D6903 testing?

ASTM D6903 is most useful when a laboratory must compare release behavior from antifouling formulations under the same controlled conditions. Although the method can be used to support product development and quality-control activities, its results should not be given as direct environmental release rates nor treated as standalone evidence in the context of an environmental risk assessment or regulatory release limits.

• Coating manufacturers compare candidate antifouling formulations and their organic-biocide release profiles.

• R&D teams examine how formulation changes affect release during prolonged laboratory exposure

• Researchers study release behavior and the factors that influence biocide availability from antifouling film.

• Quality teams compare coating batches or formulations when release characteristics form part of an internal specification or development program.

• Product developers use controlled release data when screening antifouling systems before field or service-performance studies.

What materials can be tested under ASTM D6903?

The standard covers antifouling coatings that contain an organic biocide and includes analytical methods for DCOIT, zinc pyrithione, copper pyrithione, and CDMTD. In this procedure, the technician paints the coating onto release-rate cylinders and then tests several cylinders together. If the coating also contains copper, the laboratory is able to measure the copper release at the same time using ASTM D6442.

Why is ASTM D6903 Important?

As part of their designed function, antifouling coatings discharge active compounds into the surrounding water. Because the release rate can vary over time and under different environmental conditions, manufacturers and researchers need a reliable method to compare formulations before drawing conclusions about performance. ASTM D6903 meets this requirement by using defined coated specimens, substitute ocean water, and controlling the pH, temperature, salinity, and hydrodynamics, taking scheduled measurements and carrying out chemical analysis. The method sets out a release pattern after a minimum exposure period of 45 days under the specified laboratory conditions.

ASTM D6903 Testing Process and Data Collection

Technicians begin testing by preparing the coated cylinder and checking the testing equipment to ensure the necessary conditions are stable. After conditioning, the technician places the coated cylinder into the measuring container, which contains substitute ocean water. The rotating device rotates the specimen at the required speed for the given period of time. When rotation is finished, the technician takes a water sample for biocide analysis, then returns the specimen to the holding tank. The laboratory performs this procedure at regular intervals and records the biocide concentration to calculate the release rate.

Common Challenges and Troubleshooting

Visible light is the first hazard, since photosensitive biocides decompose when exposed to it. That is why the laboratories protect the storage tank, the containers, and the samples from visible light. Since the method is capable of detecting the biocide at the parts-per-billion level, small errors are significant. Reagent impurities can cause the results to be inaccurate, which is why the standard includes acceptability criteria and a limit of quantitation for the analytical method. Moreover, the temperature, pH, salinity, and flow have to remain constant, as the release is dependent on them. Given that the exposure tests last for weeks, a single failure in control can ruin the entire series. Lastly, the analysts must not treat the figures as an in-service release.

• The quantity of the required biocide released per unit area of coating each day, given in μg/cm²/day.
• Generally, analysts work out each rate using the formula R = (C × V × 24) / (T × A).
• In the equation, C stands for the concentration of the biocide in μg/L, V for the volume of the water in L, T for the rotation time in hours, and A for the coated area in cm².
• The rates in the series give rise to a release profile which illustrates how the rate of release varies as the coating ages over a period of at least 45 days.
• An analytical method with a lower limit of quantitation lower than that required for the rates below the lower quantitation limit is needed.
• Analysts only compare the profiles of different coatings when the tests have been carried out under the same conditions.
• The laboratory values are higher than those in the in-service release, with the difference in the copper data being about 30 times.
• People should not use the numbers directly when carrying out risk assessments, making loading estimates, or setting regulatory limits unless they have been adjusted by appropriate correction factors.
• The report should include the biocide, the analytical method, and the exposure conditions together with the rates.

FAQ 

What property does ASTM D6903 measure?

The test determines how quickly organic biocide escapes from an antifouling coating when substitute ocean water is used. The laboratory gives the rate in μg cm⁻² d⁻¹ at each sampling point.

What biocides does the standard include?

The standard includes analytical methods for DCOIT (4, 5-dichloro-2-n-octylisothiazolin-3-one), CuPT (copper pyrithione), ZPT (zinc pyrithione), and CDMTD (N-cyclopropyl-N′-(1,1-dimethylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine).

How long does ASTM D6903 testing take?

The method requires a minimum exposure period of 45 days; the entire project may take longer because the laboratory also needs time to prepare the coating, perform scheduled chemical analysis, complete calculations, conduct quality checks, and prepare the reports.

Link to ASTM D6903 Standard