1. Salt spray corrosion

Corrosion is the destruction or deterioration of materials or their properties caused by the environment. Most corrosion occurs in the atmospheric environment, which contains corrosive components and factors such as oxygen, humidity, temperature changes, and pollutants. Salt spray corrosion is a common and most destructive atmospheric corrosion. The salt spray mentioned here refers to the chloride atmosphere, and its main corrosive component is the chloride salt in the ocean - sodium chloride, which mainly comes from the ocean and inland saline-alkali areas. The corrosion of the surface of metal materials by salt spray is caused by the chloride ions contained in it penetrating the oxide layer and protective layer of the metal surface and causing an electrochemical reaction with the internal metal. At the same time, chloride ions contain a certain amount of hydration energy and are easily squeezed out by the pores and cracks adsorbed on the metal surface and replace the oxygen in the chloride layer, turning insoluble oxides into soluble chlorides, turning the passivated surface into active surface. Cause extremely bad adverse reactions to the product.

2. Salt spray test and its connection with practice

Salt spray test is an environmental test that mainly uses artificial simulated salt spray environmental conditions created by salt spray testing equipment to evaluate the corrosion resistance of products or metal materials. It is divided into two categories, one is natural environment exposure test, and the other is artificial accelerated simulated salt spray environment test. The artificial simulated salt spray environment test is to use a test equipment with a certain volume space - a salt spray test chamber, to use artificial methods in its volume space to create a salt spray environment to assess the salt spray corrosion resistance performance and quality of the product. . Compared with the natural environment, the salt concentration of chloride in the salt spray environment can be several or dozens of times that of the general natural environment, which greatly increases the corrosion rate. Conduct a salt spray test on the product and obtain the results. The time is also greatly shortened. For example, if a product sample is tested in a natural exposure environment, it may take a year for it to corrode, but if tested under artificially simulated salt spray environment conditions, similar results can be obtained in just 24 hours. Artificial simulated salt spray test also includes neutral salt spray test, acetate salt spray test, copper salt accelerated acetate salt spray test, and alternating salt spray test.

(1) Neutral salt spray test (NSS test) is the earliest accelerated corrosion test method with the widest application field. It uses 5% sodium chloride salt solution, and the pH value of the solution is adjusted to the neutral range (6-7) as the solution for spraying. The test temperature is all 35℃, and the sedimentation rate of salt spray is required to be between 1~2ml/2000px2.h.

(2) The acetate salt spray test (ASS test) is developed on the basis of the neutral salt spray test. It adds some glacial acetic acid to a 5% sodium chloride solution to reduce the pH value of the solution to about 3, making the solution acidic, and the resulting salt spray also changes from neutral salt spray to acidic. Its corrosion rate is about 3 times faster than the NSS test.

(3) The copper salt accelerated acetate salt spray test (CASS test) is a rapid salt spray corrosion test recently developed abroad. The test temperature is 50°C. A small amount of copper salt-copper chloride is added to the salt solution to strongly induce corrosion. . Its corrosion rate is approximately 8 times that of the NSS test.

(4)The alternating salt spray test is a comprehensive salt spray test, which is actually a neutral salt spray test plus a constant humidity and heat test. It is mainly used for cavity-type complete machine products. Through the penetration of the moist environment, salt spray corrosion occurs not only on the surface of the product, but also inside the product. It alternately converts the product under two environmental conditions: salt spray and humid heat, and finally assesses whether there are any changes in the electrical and mechanical properties of the complete product.

3. Salt spray test standards and judgment of test results

Standards are uniform provisions for repetitive things and overviews. Salt spray test standards are clear and specific provisions on salt spray test conditions, such as temperature, humidity, sodium chloride solution concentration and PH value. In addition, technical requirements are put forward for the performance of salt spray test chambers. The salt spray test standard used for the same product should be selected based on the characteristics of the salt spray test, the corrosion rate of the metal, and the sensitivity to salt spray. Several salt spray test standards are introduced below, such as GB/T2423.17-1993 "Basic Environmental Testing Procedures for Electrical and Electronic Products Test Ka: Salt Spray Test Method", GB/T2423.18-2000 "Environmental Testing of Electrical and Electronic Products Part 2" : Test test Kb: salt spray, alternating (sodium chloride solution)", GB5938-86 "Test methods for corrosion resistance of metal coatings and chemically treated layers of light industrial products", GB/T1771-91 "Resistance of paints and varnishes to medium Determination of Resistant Salt Spray Performance".

The purpose of the salt spray test is to assess the salt spray corrosion resistance of products or metal materials. The judgment of the salt spray test results is a judgment on the product quality. Whether its judgment results are correct and reasonable is a correct measure of the salt spray resistance of the product or metal. The key to corrosion quality. The judgment methods of salt spray test results include: rating judgment method, weighing judgment method, corrosion product appearance judgment method, and corrosion data statistical analysis method. The rating judgment method divides the percentage of the corrosion area to the total area into several levels according to a certain method, and uses a certain level as the basis for qualification judgment. It is suitable for evaluation of flat samples;

The weighing judgment method is a method of weighing the sample before and after the corrosion test, and calculating the weight lost by corrosion to evaluate the corrosion resistance quality of the sample. It is especially suitable for assessing the corrosion resistance quality of certain metals;

The corrosion product appearance determination method is a qualitative determination method. It determines whether the product has corrosion phenomena after the salt spray corrosion test. This method is mostly used in general product standards;

The corrosion data statistical analysis method provides a method for designing corrosion tests, analyzing corrosion data, and determining the confidence level of corrosion data. It is mainly used to analyze and count corrosion situations, rather than specifically used to determine the quality of a specific product.

There are three types of salt spray tests: neutral salt spray test (NSS), acetic salt spray (AASS) and copper accelerated acetate salt spray (CASS, also known as copper chloride acetate salt spray) test. The most widely used is Neutral salt spray test. The basic content of the salt spray test is to spray 5% sodium chloride aqueous solution in the test chamber at 35 degrees Celsius to simulate the accelerated corrosion method of seawater environment. The length of its resistance time determines the quality of the corrosion resistance. The volume of the salt spray box is not less than 012 m3, preferably not less than 014 m3, the temperature inside the box is 35℃±2℃, the spray pressure is 70~170kPa, there are at least two salt spray collectors, and the collection area is 80 cm each. 2. Test surface 15~30° to the vertical direction to allow the salt spray to settle freely on the surface to be measured. It cannot be sprayed directly. The test bracket is made of glass or plastic. The test pieces cannot contact each other or drip with each other. The temperature inside the box is 35℃±2℃. Each collector collects solution (1～2) mLöh.

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