Galvanized bolts are widely used in harsh environments such as the ocean and industry, and their corrosion resistance is a key factor in ensuring their long-term stable performance. Salt spray testing, as a standardized evaluation method, can simulate harsh corrosion environments to verify the protective effect of the galvanized layer. This article will analyze the purpose, principle, standard, steps and influencing factors of the salt spray test in detail, and introduce this important testing process in detail.
1. Test Purpose
The core purpose of the salt spray test is to evaluate the corrosion resistance of galvanized bolts by accelerating the corrosion process to ensure that they can maintain excellent performance under long-term exposure to harsh conditions such as marine and industrial environments. Specific goals include:
（1）Evaluate corrosion resistance: simulate salt spray environment and accelerate the corrosion process to confirm that the galvanized layer can effectively prevent the substrate (such as steel) from rusting.
（2）Quality control: ensure that the production process meets the requirements by testing the uniformity, thickness and passivation effect of the galvanized layer.
（3）Comparative performance: provide a basis for comparing the corrosion resistance of galvanized bolts from different manufacturers or batches.
2. Test Principle
The salt spray test accelerates the corrosion process by exposing the bolts to salt spray containing sodium chloride. The specific principles are as follows:
（1）Environmental simulation: The bolt sample is placed in a salt spray chamber, and a 5% sodium chloride solution is sprayed in the chamber. A high salt spray environment is formed by continuous spraying to simulate the salt spray environment of the ocean.
（2）Sacrificial anode mechanism: The zinc layer acts as a sacrificial anode, corrodes first, and delays the corrosion of the steel. During the test, the formation time and area of the white rust (corrosion product of zinc) of the zinc layer and the red rust (corrosion product of iron) of the substrate are observed.
3. Test standards and classifications
There are many standards and classifications for salt spray testing, and different test standards are suitable for different application requirements. Commonly used test standards include:
（1）ASTM B117 (American standard): commonly used in neutral salt spray tests (NSS), and is one of the most widely used standards.
（2）ISO 9227 (International standard): covers multiple types such as neutral salt spray (NSS), acetic acid salt spray (ASS), and copper accelerated salt spray (CASS).
（3）GB/T 10125 (Chinese standard): equivalent to ISO 9227, suitable for domestic galvanized bolt corrosion resistance testing.
Test steps
Salt spray test usually includes the following steps:
1. Sample preparation: First, the bolt surface needs to be cleaned to remove oil and other contaminants to ensure the accuracy of the test results.
2. Parameter setting: According to the standard requirements, configure the concentration, temperature, pH value and other parameters of the salt solution to ensure that the test environment meets the standard requirements.
3. Exposure test: Put the prepared bolt sample into the salt spray box and spray it continuously. The test time is usually 24 hours, 72 hours or 240 hours.
4. Result evaluation: By checking the corrosion on the bolt surface, record the time and area of white rust and red rust, and determine whether it is qualified according to the standard.
5. Result judgment
According to the test results, the corrosion resistance of galvanized bolts can be judged by the following standards:
（1）Qualified standards:
o Ordinary galvanized bolts: no red rust (base material corrosion) within 72 hours, only slight white rust is allowed.
o Hot-dip galvanized bolts: no red rust for more than 240 hours, and the white rust area shall not exceed 5%.
（2）Failure sign: When the area of red rust or white rust exceeds the standard limit during the test, it means that the galvanized bolt fails to meet the corrosion resistance requirements and needs to re-evaluate the process or select a more suitable product.
6. Influencing factors
The results of the salt spray test may be affected by many factors:
(1) Coating quality: The thickness, uniformity and integrity of the passivation film of the coating will directly affect the corrosion resistance. Generally speaking, the corrosion resistance of hot-dip galvanized bolts is better than that of electro-galvanized bolts.
(2) Process defects: If there are pores, scratches or missed plating areas in the coating, it will accelerate the corrosion process and affect the test results.
(3) Test conditions: Factors such as salt solution concentration, temperature and humidity need to be strictly controlled to ensure the scientificity and accuracy of the test.
7. Application scenarios
Salt spray testing is widely used in the following industries:
(1) Construction industry: Bolts used for building structures, facilities, etc. that need to be exposed to harsh environments for a long time.
(2) Automobile industry: Bolts exposed to humid and salt spray environments, especially for automobile chassis, engines and other parts.
(3) Bridges and marine engineering: Bolts that are exposed to the marine environment for a long time need to have high corrosion resistance.
Through salt spray testing, manufacturers can optimize the galvanizing process, and users can choose suitable products based on the test results to ensure long-term safety.
8. Limitations
Although salt spray testing can provide a simulation of accelerated corrosion, it still has certain limitations:
(1) Accelerated and natural corrosion: Salt spray testing in the laboratory accelerates the corrosion process, but may not fully simulate the corrosion conditions in the real environment.
(2) Single factor: Salt spray testing only simulates salt spray corrosion, while the actual environment may have complex factors such as humidity and temperature fluctuations, which have a more complex impact on corrosion performance.
Summary
Salt spray testing provides a scientific evaluation method for the corrosion resistance of galvanized bolts, helping manufacturers to verify and optimize the galvanizing process, while providing users with a reliable corrosion resistance reference. By understanding and applying salt spray testing, the industry can more accurately meet the needs of corrosion resistance in material selection and design.
Post time: Feb-18-2025