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D1735-21 – Standard Test Method Technical Guide
📋 Introduction and Scope
ASTM D1735-21 provides a standardized practice for evaluating the water resistance of coatings using a water fog (fog spray) apparatus. This method is designed to assess how coatings perform under a controlled condensing fog environment, similar in principle to the well-known salt spray (fog) apparatus outlined in Practice B117.
The scope of this practice is specifically limited to the principles and operating procedures of the water fog test chamber. It does not prescribe specimen preparation, specific test durations, or the evaluation of results, deferring these to a host of complementary ASTM standards. Alternative practices for testing water resistance include Practices D870 (Water Immersion), D2247 (100 % Relative Humidity), and D4585 (Controlled Condensation).
💡 Key Operational Note: Because this practice focuses solely on operational procedures, users must carefully select the appropriate supporting standard for evaluation. The choice between D1735 and alternative methods (D870, D2247, D4585) should be driven by the intended end-use environment of the coating.
⚙️ Test Apparatus and Procedure
Coated specimens are placed within an enclosed chamber specifically designed to permit the free settling of fog on all exposed surfaces. The environment within the chamber is precisely controlled to ensure reproducible exposure conditions.
The standard stipulates that the temperature of the chamber is usually maintained at 38 °C (100 °F). The severity of the exposure is primarily varied by selecting the total duration of the test. Water permeates the coating at rates that are dependent upon the specific characteristics of the coating material being tested.
| 🟦 Parameter | 📏 Specification / Condition | 📐 Notes |
|---|---|---|
| Chamber Temperature | 38 °C (100 °F) | Usually maintained for standard testing protocols. |
| Water Quality | Reagent Water (per D1193) | Critical for preventing external contamination artifacts. |
| Specimen Placement | Free settling of fog on all specimens | Specimens must not shield each other from the fog. |
| Test Duration | Varies (e.g., 48, 96, 168, 500 hours) | Determined by applicable material specification or user criteria. |
📊 Significance and Evaluation of Results
Water is a primary agent in the degradation of organic coatings. Knowledge of how a coating resists water is helpful in predicting its service life. Failure in water fog tests can arise from a deficiency in the coating itself, improper surface preparation, or unfavorable application conditions.
After exposure, specimens are evaluated for specific failure modes. The standard references specific test methods for quantifying these changes, allowing for objective comparison and reporting of results such as blistering, loss of adhesion, softening, and color change.
⚡ Critical Evaluation Note: The water fog test evaluates water permeation and its effects. This environment is distinct from total immersion (D870) or 100% condensing humidity (D2247 and D4585). The user must evaluate results within the context of this specific accelerated test method.
| 🎯 Failure Mode | ⚡ Relevant Evaluation Standard |
|---|---|
| Blistering | D714 (Test Method for Evaluating Degree of Blistering) |
| Rusting | D610 (Practice for Evaluating Degree of Rusting) |
| Adhesion Loss | D3359 (Tape Test) / D4541 (Pull-Off Strength) |
| Color Change | D2616 (Gray Scale Evaluation) |
| Softening / Hardness | D3363 (Film Hardness by Pencil Test) |
❓ Frequently Asked Questions
🔍 What is the main difference between D1735 (Water Fog) and D4585 (Controlled Condensation)?
While both test water resistance, D4585 typically uses a heated water bath to create condensing humidity directly on the panel, whereas D1735 uses a spray nozzle within an enclosed chamber to create a fog that settles on the specimens. The mode of water contact (fog vs. condensation) can yield different results depending on the coating chemistry and application.
💡 Does D1735-21 include pass/fail criteria?
No. The standard explicitly states it is limited to the methods of obtaining, measuring, and controlling the conditions and procedures of water fog tests. It does not specify the evaluation of results. The user must establish acceptance criteria based on the specific coating requirements and use the referenced standards (D610, D714, D3359, etc.) to quantify the test outcomes.
⚡ What type of water is required for testing per D1735?
ASTM D1735 requires the use of Reagent Water conforming to Specification D1193. The purity of the water is critical to ensure that any observed failure is due to the coating’s resistance to water itself, and not an artifact of water contamination (e.g., dissolved salts or minerals) that could skew the test results.
📌 How should test panels be prepared for this practice?
Panel preparation is not covered by D1735. Users should refer to Practices D609 for standard preparation of cold-rolled steel panels or D1730 for aluminum panels. The application of the coating film itself should follow Practices D823 to ensure uniform thickness and consistency across the test specimens.