Reasons why salt spray testing is unsuitable for evaluating the rust prevention of galvanized products
Is salt spray testing an inappropriate method for evaluating hot-dip galvanized products? What is the ideal rust prevention test and what are the acceptance criteria? If the customer insists on using salt spray testing as the acceptance standard, how long should the plated parts be placed in a general indoor storage environment (constant temperature and humidity environment) to be more favorable to salt spray testing?
Rust prevention tests for galvanized products are not suitable for evaluation using salt spray tests because galvanized products rely on the formation of a stable oxide layer on their surface to prevent corrosion in natural environments. Salt spray tests, however, continuously damage the oxide layer before it has stabilized, making them unsuitable for evaluation.
Reasons why the surface of hot-dip galvanized steel is unsuitable for salt spray testing
The surface of hot-dip galvanized steel is usually not suitable for salt spray testing. When hot-dip galvanized steel is exposed to the atmosphere, zinc carbonate will form on its surface. Zinc carbonate effectively protects the zinc coating and reduces the corrosion rate of the zinc coating. However, zinc carbonate cannot be produced in salt spray tests on hot-dip galvanized steel. Therefore, salt spray tests are usually not used to evaluate the corrosion resistance and service life of hot-dip galvanized steel in different environments. Coatings applied to the surface of hot-dip galvanized steel can be tested using salt spray testing methods.
Comments from experts on the ASTM B117 salt spray test on the American Corrosion Doctor website:
ASTM B117 is the earliest and most widely used salt spray test standard. This standard mainly describes the sample specimens placed in a sealed test chamber at a specific position and angle for salt spray testing. The concentration of sodium chloride solution can range from 3.5% to 20%, and the test chamber can have a large space design. Although salt spray testing is widely used in standards, it is not related to usage characteristics. Hot water is vaporized into hot and humid mist by a compressor and bubble tower, and salt water is vaporized into corrosive salt mist by a nozzle using a gravity feed system, establishing a 100% relative humidity exposure condition area. For low humidity, forced air needs to be passed directly through the heater to the test chamber via a blower.
Questions from American corrosion expert Robert Baboian about salt spray testing evaluation
When used correctly, the salt spray test is one of the most valuable corrosion tests in the world. It has impacted all industries, and in various industries, the salt spray test has very high value in terms of quality control and material property comparison, such as the automotive, aircraft, transportation, and infrastructure industries that use paint. Salt spray testing is widely used in paint coating systems, and it is extremely valuable when used correctly for quality control and material property comparison. For example, periodic testing using ASTM B117 in various industries is extremely valuable because it is closer to actual usage conditions. Robert Baboian questions whether salt spray testing is really that good?
When attempting to evaluate the corrosion performance of automotive parts after salt spray testing, several testing problems with ASTM B117 were found. Several studies have indicated that ASTM B117 cannot provide a correct test for aluminum alloy coatings, or cannot predict the damage of aluminum-zinc alloy coatings on cold-rolled steel sheets or any other similar coatings.
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