Application of salt spray test in precision casting
Salt spray testing is a type of environmental test that assesses the corrosion resistance of a product or metallic material primarily using artificially simulated salt spray environmental conditions created by salt spray test equipment. The accelerated standardized test method is one of the well-established and widely used procedures for testing and comparing corrosion resistance in constant environments, and the test measures the corrosion resistance of steel and other materials exposed to salt spray or fog.
I. Introduction to salt spray test
Salt spray test is divided into natural environment exposure test and artificial accelerated simulation of salt spray environment test two categories. Artificial simulation of salt spray environment test is the use of a certain volume of space with a salt spray test equipment, a salt spray test chamber, in its volume of space with artificial methods, resulting in salt spray environment to the product of salt spray corrosion resistance quality assessment. It is compared with the natural environment, the salt concentration of chloride salt fog environment, can be several times the general natural environment or dozens of times the content of salt spray, so that the corrosion rate is greatly increased, salt spray test on the product, the results of the time is also greatly reduced. Such as in the natural exposure environment for a product sample test, to be its corrosion may take 1 year, while in the artificial simulation of salt spray environment conditions, as long as dozens of hours or even shorter, can get similar results.
II. Salt spray test classification
Artificial simulation of the main methods of salt spray test, including neutral salt spray test (NSS test), acetate spray test (ASS test), copper salt accelerated acetate spray test (CASS test) and alternating salt spray test.
1) Neutral salt spray test is the earliest and most widely used in one of the accelerated corrosion test method, which uses 5% sodium chloride saline solution, solution PH value adjusted in the neutral range (6.5 ~ 7.2) as a spray with the solution, the test temperature are taken to 35 ℃, the requirements of the salt spray settling rate of 1 ~ 2mL/80cm2.h. The test is carried out by a team of experts, who have been working with the company for many years.
2)Acetic acid salt spray test is developed on the basis of neutral salt spray test, it is added some glacial acetic acid in 5% sodium chloride solution, so that the PH value of the solution is reduced to 3.0-3.1, and the solution becomes acidic, and the final salt spray formed is also turned into acidic by neutral salt spray, and the corrosion rate of it is about 3 times faster than that of NSS test.
3) Copper salt accelerated acetic acid salt spray test is a newly developed foreign rapid salt spray corrosion test, the test temperature of 50 ℃, salt solution with a small amount of copper salt – copper chloride, strongly induced corrosion, its corrosion rate is about 8 times faster than the NSS test.
4) Alternating salt spray test is a comprehensive salt spray test, it is actually neutral salt spray test plus constant humidity heat test. It is mainly used for cavity-type whole products, through the penetration of the humid environment, so that the salt spray corrosion is not only generated on the surface of the product, but also inside the product. It is the product in the salt spray and humid heat two environmental conditions alternately, and finally assess the electrical and mechanical properties of the whole product with or without changes.
III. Salt spray corrosion mechanism
Salt spray corrosion of metal materials, mainly conductive salt solution penetrates into the metal internal electrochemical reaction, the formation of “low potential metal – electrolyte solution – high potential impurities” micro-battery system, electron transfer occurs, as the anode of the metal appears to be dissolved, the formation of a new compound that is corrosive. New compounds that corrosive material. Salt spray corrosion damage process plays a major role in the chlorine ion. It has a strong penetration ability, easy to penetrate the metal oxide layer into the metal, destroying the passivity of the metal. At the same time, the chloride ion has a very small hydration energy, easy to be adsorbed on the metal surface, replacing the oxygen in the oxide layer to protect the metal, so that the metal is damaged.
IV. Salt spray test standards Salt spray test is an important method to assess the corrosion resistance of products or metal materials, you can choose the appropriate test method and judgment according to the actual needs. The national standards for salt spray test are: GB/T 2423.17, IEC60068-2-11, ISO9227, ASTM B117, JIS-Z2371, JIS-G3141, GJB150.11A-2009, MIL-STD-810F, MIL-STD-883E, GB5938, GB/T1771 and so on. ASTM B117 was invented in 1939 and is the first internationally recognized standard for salt spray or salt mist, followed by ISO9227, JIS Z 2371 and ASTM G85 standards. Salt spray testing is one of the mature and widely used corrosion test methods worldwide.
Salt spray testing is a highly standardized corrosion test procedure in accordance with national and international standards. The performance of the test chamber, the test procedure and the test parameters, such as temperature, air pressure of the salt spray solution or pH, are pre-determined in accordance with the above standards. In order to show compliance with the standards and to ensure proper test conditions, the test parameters need to be checked frequently. As reference standards, ASTM B117 and ISO 9227 are widely accepted.What is the difference between ASTM B117 and ISO 9227?
ASTM B117 was established in 1939 and was the first internationally recognized standard for salt spray testing. The test procedure is widely accepted and for many years was considered the gold standard for corrosion testing.ASTM B117 is a non-cyclic test. This means that the material being tested is constantly exposed to salt spray. This situation is a major failure of commonly used tests because real-world weather conditions fluctuate.
The ISO 9227 standard is just as flawed as ASTM B117. This standard also exposes materials to salt spray for long periods of time. As a result, the test atmosphere is static, so the standard does not mimic real-world conditions. The main difference between the two corrosion test methods is that ISO 9227 is globally accepted, while ASTM B117 is only an American standard.
V. Salt spray test purpose and application Salt spray test purpose and application 1. The purpose of salt spray test in order to assess the quality of products or metal materials resistant to salt spray corrosion, and salt spray test results is the result of the product quality of the pronouncement of its determination of whether the results are correct and reasonable, is a correct measure of the quality of the product or the metal resistance to salt spray corrosion of the key.
2.Application of salt spray test
In order to conduct the corrosion test, the material being examined is placed in a salt spray test chamber. The workpiece is then exposed to an uninterrupted spray or fog of saline solution. Hence the name salt spray test. In addition to the test chamber, a prerequisite for a successful test procedure is that the environment does not change and remains stable during exposure
In order to perform the test procedure according to ASTM B117, the temperature needs to be maintained at 35 °C (+ 1.1 °C to 1.7 °C). In addition, the pH value of the test environment needs to be in the range of 6.5 to 7.2, and a salt atmosphere is also important. It needs to consist of 5% sodium chloride and 95% ASTM D1193 Type IV water, which is inlet at a stable, predetermined air pressure. The duration of the corrosion resistance test depends greatly on the material being examined and the coating of the metal. The test chamber needs to be constructed to the standards of the application.
VI. Advantages and disadvantages of salt spray testing
Salt spray testing offers many advantages over other corrosion testing methods. One of the main benefits of the procedure is that the test is relatively inexpensive. In addition, the duration of the test is fairly short compared to natural environments and provides rapid results. Therefore, salt spray testing is used to quickly compare expected and actual corrosion resistance. As a result, it plays an important role in quality control and is used as a means of checking the effectiveness of the production process.
However, salt spray testing has some disadvantages. It is not related to the actual performance of the material being tested. For example, it can only provide results on the expected life of a coating in salt water. In addition, it is a destructive test.
VII. Factors affecting salt spray corrosion 1. Test temperature and humidity
The critical relative humidity for metal corrosion is approximately 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquesce and form a conductive electrolyte. When the relative humidity decreases, the salt solution concentration will increase until the precipitation of crystalline salt, the corrosion rate decreases accordingly.
The higher the test temperature, the faster the salt spray corrosion rate. International Electrotechnical Commission IEC60355 “AN APPRAISAL OF THE PROBLEMS OF ACCELERATED TESTING FOR ATMOSPHERIC CORROSION” standard pointed out that: “the temperature rises 10 ℃, the corrosion rate increased by 2 to 3 times, the conductivity of the electrolyte increases by 10 to 20%”. This is because the temperature increases, the molecular movement increases, the chemical reaction speed up the result. For neutral salt spray test, most scholars believe that the test temperature is selected at 35 ℃ is more appropriate. If the test temperature is too high, the salt spray corrosion mechanism and the actual situation is different.
2. Concentration of salt solution
Concentration of salt solution on the corrosion rate of the impact of the material and the type of cover layer. Concentration of 5% below the steel, nickel, brass corrosion rate increases with the increase in concentration; when the concentration is greater than 5%, the corrosion rate of these metals but with the increase in concentration and decline. This phenomenon can be explained by the oxygen content in the salt solution, the oxygen content in the salt solution is related to the concentration of salt, in the low concentration range, the oxygen content increases with the increase in salt concentration, however, when the salt concentration increases to 5%, the oxygen content reaches relative saturation, if the salt concentration continues to increase, the oxygen content decreases accordingly. Oxygen content decreases, the depolarizing ability of oxygen also decreases that the corrosion effect is weakened. However, for zinc, cadmium, copper and other metals, the corrosion rate is always with the increase in salt concentration and increase.
3. Sample placement angle
The placement angle of the sample has a significant impact on the results of the salt spray test. Salt spray settlement direction is close to the vertical direction, the sample is placed horizontally, it is the largest projected area, the sample surface to withstand the amount of salt spray is also the most, so the corrosion is the most serious. GB/T2423.17 standard stipulates that “flat plate samples placed in a way that should be made to test the surface of the vertical direction of the 30-degree angle”.
4. pH value of salt solution
The pH value of the salt solution is one of the main factors affecting the results of the salt spray test. the lower the pH value, the higher the concentration of hydrogen ions in the solution, and the more acidic and corrosive it is, the stronger the corrosive properties. The Neutral Salt Spray Test (NSS) with a pH value of 6.5 to 7.2 shows that the corrosiveness of the Acetate Salt Spray Test (ASS) with a pH value of 3.0 is 1.5 to 2.0 times harsher than the Neutral Salt Spray Test (NSS) with a pH value of 6.5 to 7.2. The pH value of the salt solution changes due to environmental factors. For this reason, the domestic and foreign salt spray test standards on the pH range of the salt solution have been stipulated, and proposed to stabilize the pH value of the salt solution in the test process, in order to improve the reproducibility of the results of the salt spray test.
Influence of salt solution pH changes in the reasons and results: 1) caused by the salt spray test process of salt solution pH changes in the root cause of the main soluble substances from the air, the nature of these substances may be different, some dissolved in water after the acidic, some dissolved in water after the alkaline. 2) Salt spray test process, the air soluble substances dissolved into the salt solution or from the salt solution escaped from the process of the reversible process. 3) The effect of salt spray test pH range of the salt solution, and proposed ways to stabilize the test results to improve the reproducibility of the salt spray test results. (3) There are many factors affecting the change of pH value of the salt solution. For example, the nature and content of soluble substances in the air, pressure, air and salt solution contact area and contact time, etc. a. The nature and content of soluble substances in the air air contains CO2, SO2, NO2, H2S, etc., these gases dissolve in water to generate acid, so that the pH value of water is reduced. There may also be alkaline dust particles in the air, these substances dissolved in water will make the pH value of water increase. b. Atmospheric pressure gas solubility in water is directly proportional to the atmospheric pressure. c. The contact area and contact time of the air and salt solution spraying makes the salt solution into a salt spray with a diameter of 1 to 5μm fine particles. The increase in contact area makes the amount of gas dissolved into the liquid or the amount of gas escaping from the liquid both increase greatly.
5. Salt spray deposition and spraying method
The finer the salt spray particles, the larger the surface area formed, the more oxygen is adsorbed and the more corrosive it is. Traditional spraying methods include air pressure spraying method and spray tower method, the most obvious drawback is that the uniformity of salt spray deposition is poor, and the diameter of salt spray particles is large. Different spraying methods also have an effect on the pH of the salt solution.
VIII. Salt spray test judgment method
1, rating determination method
Percentage of the ratio of corrosion area to total area is divided into several levels according to a certain method, with a certain level as the basis for qualified judgment, it is suitable for flat samples for evaluation.
2、Weighing judgment method
By weighing the weight of the sample before and after the corrosion test, the weight of the corroded loss is calculated to judge the corrosion resistance of the sample, which is especially suitable for the assessment of the corrosion resistance of a certain metal.
3、Corrosive appearance judgment method
Corrosive appearance determination method is a qualitative determination method, it is salt spray corrosion test, whether the product produces corrosion phenomenon to determine the sample, the general product standards are mostly used in this method.
4、Statistical analysis of corrosion data
Corrosion data statistical analysis method provides the design of corrosion tests, analyze corrosion data, corrosion data to determine the confidence level of the method, which is mainly used to analyze, statistical corrosion, rather than specifically for a specific product quality determination.
IX. Salt spray test precautions 1, for the test with the sample plate substrate, must be thoroughly removed from rust and lubricating oil and grease. Whether by sandblasting, sanding or phosphating the substrate, be careful not to expose to humid air, in order to prevent the formation of a water film on the surface of the substrate caused by rust or thus reduce the adhesion between the coating and the substrate. 2, the key to the salt spray test is to formulate the concentration of the electrolyte solution, a variety of components of the solute should be weighed strictly according to the ratio to ensure that the accuracy of the pH. 3, otherwise it will directly affect the test results. 4, the salt spray test is the key to the preparation of electrolyte solutions. Otherwise, it will directly affect the test results.3, after preparing the coating of the sample plate, pieces, need to use the coating to seal the edge and cover the exposed parts of the substrate, otherwise, causing rust marks hanging, contamination of the plate, will bring difficulties to the assessment of the grade of the work.4, regular checking of the plate, pieces, should be to keep the plate surface in a wet state, try to shorten the time of the plate surface is exposed to the air.5, after completing the test, the surface should be made immediately after the test, an objective evaluation of the surface of the plate, including: Blistering, discoloration, rust, peeling. We can also add the test evaluation of adhesion and scratching one-side rusting distance according to customers’ requirements.
X. Salt spray test summary salt spray test is an important means of assessing the resistance of products or materials to salt spray corrosion, the test results of the scientific and rational is crucial. Salt spray test results affect the stability and consistency of many factors, to improve the effectiveness of the results of the salt spray test, test technology is the key. Test personnel not only need to have solid professional knowledge and professional skills, but also need a wealth of practical experience and a comprehensive understanding of the product, from the chemical and environmental engineering, materials, structures and processes and other multidisciplinary fields to understand the salt spray test, scientific and reasonable expression of the test results, to better select materials for the product, structural design, process selection, product transportation storage and use of the product to provide effective information to improve the product or the material’s Salt spray corrosion resistance
