What is salt spray test
What is salt spray test?
Table of Contents
- 1 What is salt spray test?
- 2 Classification of salt spray test
- 3 Standard for salt spray test
- 4 Technical analysis of salt spray test
- 5 Factors affecting the results of salt spray test
- 6 Test method of salt spray test
- 7 Judgment of salt spray test results
- 8 Function of salt spray test
- 9 Salt spray test of stainless steel
Salt spray test is a kind of environmental test which mainly uses the artificial simulated salt spray environmental conditions created by the salt spray test equipment to evaluate the corrosion resistance of products or metal materials. It can be divided into two categories, one is natural environment exposure test, the other is artificial accelerated simulated salt spray environment test. Artificial simulated salt spray environment test is to use a kind of test equipment with a certain volume space – salt spray test chamber. In its volume space, artificial method is used to create salt spray environment to assess the quality of salt spray corrosion resistance of products.
Corrosion is the destruction or deterioration of materials or their properties under the action of environment. Most of the corrosion occurs in the atmosphere, which contains oxygen, humidity, temperature changes and pollutants.
Salt spray corrosion is one of the most common and destructive atmospheric corrosion. The corrosion of metal surface caused by salt spray is due to the electrochemical reaction between chloride ions penetrating the oxide layer and protective layer of metal surface and internal metal. At the same time, chloride ion contains a certain amount of hydration energy, which is easy to be squeezed out by the pores and cracks adsorbed on the metal surface and replace the oxygen in the oxide layer, turning the insoluble oxide into soluble chloride and the passive surface into active surface.
Salt spray test is a kind of environmental test which mainly uses the artificial simulated salt spray environmental conditions created by the salt spray test equipment to evaluate the corrosion resistance of products or metal materials. It can be divided into two categories, one is natural environment exposure test, the other is artificial accelerated simulated salt spray environment test.
Artificial simulated salt spray environment test is to use a kind of test equipment with a certain volume space – salt spray test chamber (as shown in the figure). In its volume space, artificial method is used to create salt spray environment to assess the quality of salt spray corrosion resistance of products.
Compared with the natural environment, the salt concentration of chloride in the salt spray environment can be several times or dozens of times of that in the general natural environment, so that the corrosion rate is greatly improved, and the time to get the results of the salt spray test is greatly shortened. If a product sample is tested in the natural exposure environment, it may take one year for its corrosion. However, if it is tested in the artificial simulated salt spray environment, it only takes 24 hours to get similar results.
Classification of salt spray test
The artificial simulated salt spray test includes neutral salt spray test, acetate spray test, copper salt accelerated acetate spray test and alternating salt spray test.
Neutral salt spray test
It is the earliest and most widely used accelerated corrosion test method. Under normal circumstances, it uses 5% NaCl solution and the pH value is adjusted to neutral range (6.5 to 7.2) as a spray solution. The test temperature is 35 ℃, the sedimentation rate of salt spray is required to be between 1-3ml / 80cm2. H, and the sedimentation amount is generally between 1-2ml / 80cm2. H.
Acetate spray test
It is developed on the basis of neutral salt spray test. It is to add some glacial acetic acid into 5% sodium chloride solution, so that the pH value of the solution is reduced to about 3, the solution becomes acidic, and finally the salt spray formed also becomes acidic from neutral salt spray. Its corrosion rate is about three times faster than that of NSS test.
Copper salt accelerated acetate spray test
It is a kind of rapid salt spray corrosion test developed recently abroad. The test temperature is 50 ℃, and a small amount of copper salt copper chloride is added into the salt solution to strongly induce corrosion. Its corrosion rate is about 8 times that of NSS test.
Alternating salt spray test
It is a kind of comprehensive salt spray test, which is actually neutral salt spray test plus constant humidity and heat test. It is mainly used for cavity type products, through the penetration of wet environment, salt spray corrosion not only occurs on the surface of the product, but also inside the product. It alternates the product in salt spray and hot and humid environment, and finally checks whether the electrical and mechanical properties of the whole product have changed.
Standard for salt spray test
The standard of salt spray test is to specify the conditions of salt spray test, such as temperature, humidity, concentration of sodium chloride solution and pH value. In addition, it also puts forward technical requirements for the performance of salt spray test chamber. The standard of salt spray test for the same product should be selected according to the characteristics of salt spray test, the corrosion rate of metal and the sensitivity to salt spray. The following introduces several salt spray test standards, such as GB / t2423.17-2008 basic environmental test procedures for electrical and electronic products, Test Ka: salt spray test method, GB / t2423.18-2000 environmental test for electrical and electronic products Part 2: Test Kb: salt spray, alternating (sodium chloride solution), gb5938-86 corrosion resistance test method for metal coating and chemical treatment layer of light industrial products, GB / t1771-91 paints and varnishes – Determination of resistance to neutral salt spray.
The purpose of salt spray test is to assess the salt spray corrosion resistance quality of products or metal materials, and the judgment of salt spray test results is the judgment of product quality. Whether the judgment results are correct and reasonable is the key to correctly measure the salt spray corrosion resistance quality of products or metals. The judgment methods of salt spray test results include rating judgment method, weighing judgment method, corrosives appearance judgment method and corrosion data statistical analysis method. The grading method is to divide the percentage of the corrosion area to the total area into several grades according to a certain method, and take a certain grade as the basis for qualification judgment. It is suitable for the evaluation of plate samples; Weighing method is a method of weighing the weight of the sample before and after the corrosion test, and calculating the weight of the corrosion loss to evaluate the corrosion resistance quality of the sample. It is especially suitable for the assessment of the corrosion resistance quality of a certain metal; The corrosives appearance judgment method is a qualitative judgment method. It judges the samples by whether the products are corroded after the salt spray corrosion test. This method is mostly used in general product standards; The statistical analysis method of corrosion data provides a method to design corrosion test, analyze corrosion data and determine the confidence level of corrosion data. It is mainly used to analyze and count corrosion situation, rather than to determine the quality of a specific product.
Technical analysis of salt spray test
The harm of corrosion
Salt spray corrosion will destroy the metal protective layer, make it lose its decoration and reduce the mechanical strength; Some electronic components and electrical circuits are interrupted due to corrosion, especially in the vibration environment; When the salt spray falls on the insulator surface, the surface resistance will decrease; After the insulator absorbs the salt solution, its volume resistance will decrease by four orders of magnitude; The moving parts of mechanical parts or moving parts increase the friction due to the generation of corrosion, resulting in the moving parts being stuck.
Mechanism of corrosion
The corrosion of metal materials by salt spray is mainly due to the penetration of conductive salt solution into the metal and electrochemical reaction, forming a “low potential metal electrolyte solution high potential impurity” micro battery system, which causes electron transfer and dissolves the metal as anode to form a new compound, namely corrosive substance. The metal protective layer and the organic material protective layer are the same. When the salt solution as the electrolyte penetrates into the interior, a micro battery with metal as the electrode and metal protective layer or organic material as the other electrode will be formed.
Chloride ion plays a major role in the process of salt spray corrosion. It has a strong penetration ability, easy to penetrate the metal oxide layer into the metal, destroy the metal blunt state. At the same time, chloride ion has a very small hydration energy, which is easy to be adsorbed on the metal surface and replace the oxygen in the oxide layer to protect the metal.
In addition to chloride ion, the mechanism of salt spray corrosion is also affected by oxygen dissolved in salt solution (essentially the salt film dissolved on the surface of the sample). Oxygen can cause the depolarization process of the metal surface and accelerate the dissolution of the anode metal. Because of the continuous spray during the salt spray test, the salt film on the surface of the sample continuously settles, so that the oxygen content is always kept near saturation. The formation of corrosion products causes the volume expansion of the salt solution penetrating into the metal defects, thus increasing the internal stress of the metal, causing stress corrosion and bulging of the protective layer.
Factors affecting the results of salt spray test
The main factors influencing the results of salt spray test include temperature and humidity, concentration of salt solution, angle of sample placement, pH value of salt solution, sedimentation of salt spray and spray method.
Test temperature and humidity
Temperature and relative humidity affect the corrosion of salt spray. The critical relative humidity for metal corrosion is about 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquescence and form an electrolyte with good conductivity. When the relative humidity decreases, the concentration of salt solution will increase until crystalline salt is precipitated, and the corrosion rate will decrease accordingly.
The higher the test temperature is, the faster the salt spray corrosion rate is. According to IEC 60355:1971 “an approximate of the problems of accelerated testing for atmospheric corrosion”, the conductivity of electrolyte increases by 10-20% when the temperature increases by 10 ℃. This is because the temperature increases, the molecular motion intensifies, and the chemical reaction speed accelerates. For neutral salt spray test, most scholars think that the test temperature is 35 ℃. If the test temperature is too high, the salt spray corrosion mechanism is quite different from the actual situation.
Concentration of salt solution
The effect of salt solution concentration on corrosion rate is related to the type of material and coating. When the concentration is below 5%, the corrosion rate of steel, nickel and brass increases with the increase of concentration; When the concentration is more than 5%, the corrosion rate of these metals decreases with the increase of concentration. The above 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 the salt. In the low concentration range, the oxygen content increases with the increase of the 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 will decrease accordingly. With the decrease of oxygen content, the depolarization ability of oxygen also decreases, that is, the corrosion is weakened. But for zinc, cadmium, copper and other metals, the corrosion rate always increases with the increase of salt solution concentration.
Sample placement angle
The angle of sample placement has a significant effect on the results of salt spray test. The deposition direction of the salt spray is close to the vertical direction. When the sample is placed horizontally, its projection area is the largest, and the amount of salt spray on the sample surface is the largest, so the corrosion is the most serious. The results show that the weight loss per square meter is 250 g when the steel plate is at a 45 degree angle to the horizontal line, and 140 g per square meter when the steel plate plane is parallel to the vertical line. The GB / t2423.17-93 standard stipulates that “the placement method of flat sample should make the test plane form a 30 degree angle with the vertical direction.”
PH value of salt solution
The pH value of salt solution is one of the main factors affecting the results of salt spray test. The lower the pH value, the higher the concentration of hydrogen ion in the solution, the stronger the acidity and the stronger the corrosiveness. The results of salt spray test on Fe / Zn, Fe / CD, Fe / Cu / Ni / Cr Electroplating parts show that the corrosion of salt solution with pH value of 3.0 in acetate spray test (ASS) is 1.5-2.0 times more severe than that of neutral salt spray test (NSS) with pH value of 6.5-7.2.
Due to the influence of environmental factors, the pH value of salt solution will change. In order to improve the reproducibility of salt spray test results, the pH range of salt solution is specified in the salt spray test standards at home and abroad, and the method of stabilizing the pH value of salt solution in the test process is proposed.
Causes and results of pH change of salt solution
1) The change of pH value of salt solution in the process of salt spray test is mainly caused by the soluble substances in the air. The properties of these substances may be different, some are acidic when dissolved in water, and some are alkaline when dissolved in water;
2) In the process of salt spray test, the process of soluble substances in the air dissolving into or escaping from the salt solution is a reversible process. The pH value of the salt solution will decrease when the dissolved substances are added, but increase when the released substances are added. The pH value of the salt solution will decrease when the rate of decrease is equal to the rate of increase and the rate of dissolution is greater than the rate of escape. On the contrary, the pH value of salt solution increases. When the dissolution and escape rates are equal, the pH value remains unchanged.
3) There are many factors that affect the pH value of salt solution. For example, the nature and content of soluble substances in air, pressure, contact area and contact time between air and salt solution, etc.
a. Properties and contents of soluble substances in air
The air contains CO2, SO2, NO2, H2S and so on. When these gases are dissolved in water, they will form acidic substances and reduce the pH value of water. There may also be alkaline dust particles in the air, which will increase the pH value of water if dissolved in water.
b. Atmospheric pressure
The solubility of gas in water is directly proportional to atmospheric pressure. At 0 ℃, 0.355 g CO2 can be dissolved in 100 ml water at 1 atm, while 0.670 g CO2 can be dissolved in 100 ml water at 2 ATM. When using compressed air spray, because of the increase of atmospheric pressure, the dissolved amount of CO2 and other acidic substances in the air increased, and the pH value of the salt solution decreased. This process is contrary to the process of CO2 escaping from the salt solution after the spray is affected by the temperature drop.
c. Contact area and time of air and salt solution
The spray turned the salt solution into a diameter of 1~5. μ M fine particles. With the increase of contact area, the amount of gas dissolved in or escaped from the liquid increases greatly. When the conditions (such as pressure, temperature, etc.) that affect the gas dissolving into and escaping from the liquid remain unchanged, the dissolving and escaping velocity will eventually reach the equilibrium state. Before reaching the equilibrium state, with the increase of time, the amount of dissolved (or escaped) will also increase.
- ① The pH value of salt solution stored in closed container does not change with the increase of storage time. The reason is that there is no contact with air.
- ② The pH value of the salt solution stored in the uncovered culture dish decreased significantly with the increase of gas-liquid contact time. Obviously because of the large contact area with the air.
- ③ In the environment containing alkaline substances, the pH value of the salt solution in the uncovered container increases with the increase of storage time.
The finer the salt spray particles, the larger the surface area, the more oxygen absorbed and the stronger the corrosiveness. More than 90% of salt spray particles in nature are less than 1 micron in diameter. The research results show that the oxygen absorbed on the surface of 1 micron salt spray particles is relatively balanced with the oxygen dissolved in the particles. No matter how small the salt spray particles are, the amount of oxygen adsorbed will not increase.
The traditional spray methods include air pressure injection 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 larger. The ultrasonic atomization method uses the principle of ultrasonic atomization to atomize the salt solution directly into the salt spray and diffuse into the test area, which solves the problem of poor uniformity of salt spray deposition, and the diameter of salt spray particles is smaller. Different spraying methods also affect the pH value of salt solution.
The ultrasonic atomization method without using compressed air had little effect on the pH value of the salt solution, while the pH value of the salt solution changed significantly by using the air pressure injection method and the spray tower method.
1) Working principle of ultrasonic atomization
The working principle of ultrasonic atomization is to use the ultrasonic generator and transducer to generate self-excited oscillation and radiate strong ultrasonic wave to the water. The ultrasonic wave transmits the salt solution to be atomized in the atomization cup through water and semi permeable membrane, so that the microbubbles in the salt solution vibrate under the action of sound field. When the sound pressure reaches a certain value, the microbubbles expand rapidly and then close suddenly, The shock wave is generated when the microbubble is closed. This dynamic process of expansion, closure and oscillation is called acoustic cavitation. Under the action of acoustic cavitation, the liquid disperses in the gas phase and forms a fine mist on the liquid surface. Driven by the flowing gas, the fine mist continuously flows out of the atomizing cup to realize ultrasonic atomization. In the whole process, there is only physical reaction, but no chemical reaction.
2) Control of salt spray deposition in ultrasonic atomization
It is easy to control the salt spray settling rate by ultrasonic atomization method. The factors influencing the salt spray settling rate include temperature, pressure, salt solution concentration, salt spray particle diameter, atomization speed, etc.
When other conditions are fixed, the settling rate of salt spray can be adjusted by adjusting the diameter of salt spray particles. The higher the ultrasonic frequency is, the finer the salt mist is, and the lower the deposition rate is. The settling rate of salt spray can be controlled by adjusting the ultrasonic frequency.
The atomization speed is closely related to the ultrasonic power, and the settling rate of salt spray can be adjusted by adjusting the power of ultrasonic generator. So the settlement rate per unit time can be controlled. The output of salt spray can also be adjusted by adjusting the air flow into the air inlet of the atomizing cup. When the air intake is large, the number of microbubbles in the liquid will increase, and it is easy to form more fine fog. At the same time, due to the increase of pressure difference, the velocity of salt fog will accelerate, and the amount of fog entering the test area will increase.
In order to prove the feasibility and superiority of ultrasonic atomization, the following two experiments will be carried out
① Feasibility test of ultrasonic atomization
The purpose of this experiment is to:
- (1) Whether the salt spray of ultrasonic atomization settles.
- (2) whether the salt spray settling rate can be controlled.
- (3) whether there are adverse physical and chemical changes after atomization of salt solution.
The ultrasonic generator atomizes the salt liquid in the atomizing cup and diffuses into the test area through the plastic hose. With the increase of the diffusion concentration, the salt fog begins to settle. The higher the concentration of salt fog, the faster the deposition. Finally, the settlement rate reaches equilibrium and tends to be stable. During the ultrasonic atomization test, the concentration of salt solution, pH value and temperature of each point in the test area all meet the requirements of salt spray standard.
② Experiment on uniformity of salt spray settlement
The purpose of this experiment is to prove that the uniformity of salt spray deposition in ultrasonic atomization method is obviously improved compared with that in pneumatic spray method. Compared with the air pressure spray method, the salt spray particles produced by the ultrasonic atomization method are fine and uniform, and their diameters can be controlled from a few microns to 20 microns μ The consistency is good. However, the salt spray particles produced by the air pressure injection method are coarse and fine, and their diameters can reach several hundred microns, resulting in uneven distribution of salt spray in the test area and reducing the effective test area.
Test method of salt spray test
The parts with electroplated and electroless plating treatment on the surface of metal substrate were tested in artificial atmosphere corrosion test and salt spray test (GB / t10125-97)
a. Test solution
The chemically pure sodium chloride is dissolved in distilled water or deionized water with a concentration of 50 ± 5g/L。 The pH value of solution can be measured by acidimeter, and the precision pH test paper calibrated by acidimeter can also be used for daily detection. The pH value of the solution can be adjusted by chemically pure hydrochloric acid or sodium hydroxide. The pH value of salt spray collecting solution in the test chamber is 6.5-7.2. To avoid nozzle clogging, the solution must be filtered before use.
The type, quantity, shape and size of the sample shall be determined according to the requirements of the coating to be tested or the product standard. If there is no standard, it can be decided through consultation with relevant parties. Before the test, the sample must be fully cleaned, and the cleaning method depends on the surface condition of the sample and the nature of the dirt. Abrasives and solvents that will etch the surface of the specimen shall not be used. After the sample is cleaned, contamination must be avoided. If the specimen is cut from the workpiece, the coating near the cutting area shall not be damaged. Unless otherwise specified, the cutting area must be protected by appropriate covering, such as paint, stone or adhesive tape.
c. Sample placement
The sample is placed in the test chamber with the test surface facing up, so that the salt spray can freely settle on the test surface. The test surface can not be directly sprayed by the salt spray, and the angle of sample placement is important. The test plane of the plate specimen is 15 ° to the vertical direction °－ thirty °， And try to be 20 °。 Samples with irregular surface (such as the whole workpiece) should also be as close as possible to the above provisions. The samples shall not contact the box or each other. The distance between the samples shall not affect the free fall of the salt spray on the tested surface. The droplets on the sample shall not fall on other samples. The sample holder is made of glass, plastic and other materials. The material for hanging the sample shall not be metal, but man-made fiber, cotton fiber or other insulating materials. The droplet on the support shall not fall on the sample.
d. Test conditions
The temperature inside the spray box is 35. ± 2℃。 The rate of salt spray settling, after 24h spray, should be 1 80cm for each solution collected, and the concentration of NaCl is 50 2ml/h. ± 10 g / L, pH 6.5-7.2. The mist passing through the sample area shall not be used again.
e. Test cycle
The test time shall be determined according to the requirements of the tested coating or product standard; If there is no standard, it can be decided by relevant parties through consultation. The recommended test time is 2, 6, 16, 24, 48, 96, 240, 480, 720 H. The spray should not be interrupted during the prescribed test period. Open the salt spray box only when it is necessary to observe the sample briefly. If the end of the test depends on the time at which corrosion begins, the specimen needs to be inspected frequently. Therefore, these samples can not be tested together with the samples with predetermined test cycle. The test of predetermined cycle can be checked according to the above cycle. However, the test surface should not be damaged during the inspection. The time for open box inspection of samples shall be as short as possible.
f. Cleaning of sample after test
After the test, take out the sample. In order to reduce the detachment of corrosion products, the samples were dried in the room for 0.5-1 h before cleaning. Then gently clean with clean flowing water not higher than 40 ℃, remove the residue of salt spray solution on the sample surface, and immediately dry it with a blower.
g. Evaluation of test results
The test results are compared with the test pieces of metal and other inorganic coatings on the metal substrate after corrosion test and the test piece rating GB / t6461-2002 technical standard and the agreement of both parties. If the test results are within the standard range, it is judged as qualified, otherwise it is judged as unqualified.
Aluminum and aluminum alloy anodized parts were tested according to Cass standard.
Copper accelerated acetate spray test (CASS) method for anodic oxidation film of aluminum and aluminum alloy.
a. Test solution
Dissolve the analytically pure sodium chloride in distilled water or deionized water to make its concentration 50 ± 5g/L。 The analytical pure copper dichloride (CuCl2 · 2H2O) was added to the sodium chloride solution to make its concentration 0.26 ± 0.02g/L（0.205 ± 0.015g/L CuCl2）。 The pH value of the solution was adjusted to 3.0-3.1 with analytical pure glacial acetic acid and sodium hydroxide. The pH value should be measured with pH meter at 25 ℃, or daily detection with precision pH test paper. The solution must be filtered before use to avoid clogging the nozzle.
b. Sample (same as B in NSS standard for metal coating)
c. Sample placement (same as C in NSS standard for metal cladding)
d. Test conditions
The temperature inside the spray box is 35. ± 2℃。 The rate of salt spray settling, after 24h spray, should be 1 80cm for each solution collected, and the concentration of NaCl is 50 2ml/h. ± 10 g / L, pH 3.0-3.1. The mist passing through the sample area shall not be used again. In order to compare the test conditions of different laboratories or different dates, nickel plate can be used for calibration.
e. Test cycle
The test time shall be determined according to the requirements of the tested coating or product standard; If there is no standard, it can be decided by relevant parties through consultation. The recommended test time is 4, 8, 16, 26, 32, 40, 48, 56, 64 and 72 hours. The spray should not be interrupted during the prescribed test period. Open the salt spray box only when it is necessary to observe the sample briefly.
f. Cleaning of sample after test
After the test, take out the sample. After natural drying for 0.5-1 h, gently clean the sample with clean flowing water not higher than 40 ℃, remove the residue of salt spray solution on the surface of the sample, and immediately dry the sample with compressed air or blower not more than 200 kPa.
g. Evaluation of test results (same as NSS standard for metal coating)
3) Test results and judgment criteria
The test results are compared with the technical standards of related products and the agreement of both parties. If the test results are within the standard range, they are qualified, otherwise they are unqualified. If not specified, only the following aspects need to be considered for routine records:
- a. Appearance after test;
- b. Appearance after removal of corrosion products;
- c. The distribution and quantity of corrosion defects such as pitting, cracks, bubbles, etc.
The above test results shall be evaluated according to the method specified in GB / t6461-2002 “grading of specimens and test pieces of metal and other inorganic coatings on metal substrates after corrosion test”.
5) Sample preservation
The sample after the test should be put in the sample bag, and the sample situation, test date, etc. should be written on the sample bag. Generally, it should be kept for more than six months.
Judgment of salt spray test results
The results of salt spray test are generally given in qualitative form rather than quantitative form. There are four specific judgment methods.
① The grading method is to divide the percentage of corrosion area to total area into several grades according to a certain method, and take a certain grade as the basis for qualification judgment. It is suitable for the evaluation of plate samples.
② Weighing method is a method of weighing the weight of the sample before and after the corrosion test to calculate the weight of the corrosion loss to evaluate the corrosion resistance quality of the sample. It is especially suitable for the assessment of the corrosion resistance quality of a certain metal.
③ The corrosives appearance judgment method is a kind of qualitative judgment method. It judges the samples by whether the products are corroded after the salt spray corrosion test. This method is mostly used in general product standards.
④ The statistical analysis method of corrosion data provides a method to design corrosion test, analyze corrosion data and determine the confidence level of corrosion data. It is mainly used to analyze and count corrosion situation, rather than to determine the quality of a specific product.
Function of salt spray test
Salt spray test is an important means to evaluate the salt spray corrosion resistance of products or materials, and the scientificity and rationality of the test results are very important. There are many factors that affect the stability and consistency of salt spray test results. To improve the effectiveness of salt spray test results, test technology is the key. Therefore, the test personnel not only need to have solid professional knowledge and skills, but also need rich practical experience and comprehensive understanding of the product. They need to understand the salt spray test from multi-disciplinary fields such as chemical and environmental engineering, materials, structure and technology, and express the test results scientifically and reasonably, so as to better provide reference for product selection, structure design, process selection, and so on The transportation, storage and use of products provide effective information to improve the salt spray corrosion resistance of products or materials.
Salt spray test of stainless steel
Salt spray test, invented in the early 20th century, is the longest used “corrosion test”. It is favored by users of corrosion-resistant materials and has become a “universal” test. The main reasons are as follows:
- ① Time saving;
- ② Low cost;
- ③ It can test a variety of materials;
- ④ The results are simple and clear, which is conducive to the settlement of commercial disputes.
In practical application, the salt spray test of stainless steel is the most well-known – how many hours can this material be tested in salt spray? Practitioners must be familiar with this issue.
Material suppliers usually use passivation treatment or improve the surface polishing level to improve the salt spray test time of stainless steel. But the most critical determinant is the composition of stainless steel itself, that is, the content of chromium, molybdenum and nickel.
The higher the content of Cr and Mo, the stronger the corrosion resistance to pitting corrosion and crevice corrosion. This corrosion resistance is expressed by the so-called pitting resistance equivalent (pre): pre = Cr + 3.3 ×% Mo。
Although nickel can not increase the resistance of steel to pitting and crevice corrosion, it can effectively slow down the corrosion rate after the corrosion process begins. Therefore, austenitic stainless steel containing nickel often has better performance in salt spray test, and its corrosion phenomenon is much less than that of low nickel ferritic stainless steel with similar pitting corrosion equivalent.
Small knowledge: Standard 304, neutral salt spray generally between 48-72 hours; The standard 316, neutral salt spray generally in 72-120 hours.
It should be pointed out that the salt spray test has significant defects in testing the performance of stainless steel. In the salt spray test, the chloride content of salt spray is very high, far beyond the real environment, so the stainless steel which can resist corrosion in the practical application environment with very low chloride content will also be corroded in the salt spray test.
Salt spray test changes the corrosion behavior of stainless steel. It can not be regarded as an accelerated test or a simulation test. The result is one-sided and has no equivalent relationship with the actual performance of the stainless steel finally put into use.
So we can use the salt spray test to compare the corrosion resistance of different types of stainless steel, but this test can only give the material rating. When selecting stainless steel materials, salt spray test alone usually can not provide sufficient information, because we lack sufficient understanding of the relationship between test conditions and practical application environment.
For the same reason, it is not possible to estimate the service life of products only based on the salt spray test of stainless steel samples.
In addition, different types of steel can not be compared, for example, we can not compare stainless steel and coated carbon steel, because the corrosion mechanism of the two materials used in the test is quite different, and the correlation between the test results and the final actual use environment is also different.
Source: Network Arrangement – China Stainless Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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