Salt Spray Test
Corrosion is the destruction or deterioration of a material or its properties caused by the action of the environment. Most of the corrosion occurs in the atmosphere. The atmosphere contains oxygen, humidity, temperature changes, and contaminants such as corrosion and corrosion factors. Salt spray corrosion is a common and most damaging atmospheric corrosion. The corrosion of the surface of the metal material by the salt spray is caused by the electrochemical reaction between the contained chloride ions penetrating the metal surface oxide layer and the protective layer and the internal metal. At the same time, the chloride ion contains a certain amount of hydration energy, which is easily adsorbed on the pores and cracks on the metal surface to displace and replace the oxygen in the oxide layer, turning the insoluble oxide into soluble chloride, and making the passive state surface into a lively surface.
The salt spray test is an environmental test that mainly uses the simulated salt spray environmental conditions created by salt spray test equipment to assess the corrosion resistance of a product or metal material. It is divided into two major categories, one for natural environmental exposure tests and the other for accelerated artificial salt spray environmental tests. Artificially simulated salt spray environmental test is to use a test equipment with a certain volume of space - salt spray test chamber, in its volumetric space with artificial methods, resulting in a salt spray environment to assess the product's resistance to salt spray corrosion quality and quality . Compared with the natural environment, the salt concentration of the chloride in the salt spray environment can be several times or several times of that of the normal natural environment, which greatly increases the corrosion rate. The salt spray test is performed on the product and results are obtained. The time has also been greatly shortened. If a product sample is tested under natural exposure conditions, corrosion may take up to 1 year. However, if the test is performed under simulated artificial salt spray conditions, similar results can be obtained as long as 24 hours.
Artificially simulated salt spray tests include 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 one of the most widely used field of accelerated corrosion test methods. In general, it uses a 5% sodium chloride aqueous solution and the pH of the solution is adjusted to a neutral range (6.5 to 7.2) as a spray solution. Test temperature is taken at 35 °C, the required salt spray settlement rate between 1 ~ 3ml/80cm2.h, settlement is generally between 1 ~ 2ml/80cm2.h.
Acetate spray test
It was developed on the basis of a neutral salt spray test. It is to add some glacial acetic acid in 5% sodium chloride solution, so that the PH value of the solution is reduced to about 3, the solution becomes acidic, and the final salt fog is also changed from neutral salt fog to acidic. Its corrosion rate is about three times faster than the NSS test.
Copper salt accelerated acetate spray test
It is a rapid salt spray corrosion test recently developed abroad. The test temperature is 50°C. Adding a small amount of copper salt, cupric chloride, into the salt solution strongly induces corrosion. Its corrosion rate is about 8 times that of the NSS test.
Alternating Salt Spray Test
It is an integrated salt spray test that is actually a neutral salt spray test plus a constant damp heat test. It is mainly used for cavity type machine products. Through the infiltration of the tide environment, the salt spray corrosion will not only be produced on the surface of the product, but also produced inside the product. It is to switch the product alternately under the conditions of salt spray and damp heat, and finally to assess whether there is any change in the electrical properties and mechanical properties of the complete product.
The salt spray test standard is a specific and specific requirement for salt spray test conditions, such as temperature, humidity, concentration of sodium chloride solution and PH value, and also provides technical requirements for the performance of the salt spray test chamber. Which kind of salt spray test standard for the same product should be selected according to the characteristics of the salt spray test and the metal corrosion rate and sensitivity to salt spray. The following describes several salt spray test standards, such as GB/T2423.17-1993 "Electrical and electronic products basic environmental testing procedures test Ka: salt spray test method", GB/T2423.18-2000 "electrical and electronic products environmental test part 2 : Test test Kb: salt spray, alternating (sodium chloride solution), GB5938-86 "Corrosion Resistance Test Method for Metal Coatings and Chemical Treatment Layers of Light Industrial Products," GB/T 1771-91 "Painting and Varnish Resistance Determination of salt spray performance." Salt spray test standard summary:
ISO 7253-1996 (Paints),BS 3900-F12-1997 (paint),BS 7479:1991,IEC 60068-2-11:1981,GB/T 10125-1997 (paint),GB 2423.17-2008,DIN 50021-1988
Acid salt spray test standards:
ASTM B368-09,ISO 9227-2006,DIN 50021-1988,BS 7479:1991
Copper ion accelerated salt spray test standard
ASTM B368-09,ISO 9227-2006,DIN 50021-1988,BS 7479:1991
Cyclic salt spray test standard
ASTM D6899-2003,ASTM G85-02e1 Annex A5,ISO 11997-1:2005,ISO 11997-2:2000
The purpose of the salt spray test is to evaluate the salt spray corrosion resistance of the product or metal material. The salt spray test result is the judgement of the product quality. Its judgment result is correct and reasonable. It is a correct measure of the product or metal salt spray resistance. The key to corrosion quality. Salt spray test results are judged by methods such as: rating determination method, weighing determination method, corrosive appearance determination method, and corrosion data statistical analysis method. The rating judgment method divides the percentage of the ratio of the erosion area to the total area into several levels according to a certain method. A certain level is used as a criterion for the determination of conformity. It is suitable for flat plate samples for evaluation. The weighing determination method passes through before and after the corrosion test. The weight of the sample is weighed, and the weight lost from the corrosion is calculated to evaluate the quality of the corrosion resistance of the sample. It is particularly suitable for assessing the corrosion resistance of a certain metal; the corrosion occurrence determination method is a qualitative determination. The method, which is based on the salt spray corrosion test, determines whether the product is corroded to determine the sample. Most of the general product standards use this method. The corrosion data statistical analysis method provides confidence in the design of corrosion tests, analysis of corrosion data, and determination of corrosion data. Degree method, it is mainly used for analysis and statistics of corrosion, not for the specific quality determination of a specific product.
Salt spray corrosion will damage the metal protective layer, causing it to lose its decorative properties and reduce its mechanical strength. Some electronic components and electrical wiring will cause interruption of the power supply line due to corrosion, especially in a vibrating environment, especially when salt spray occurs. When landing on the surface of the insulator, the surface resistance will be reduced; after the insulator absorbs the salt solution, its volume resistance will be reduced by four orders of magnitude; the moving parts of the mechanical parts or moving parts will increase the friction due to the generation of corrosive substances and cause movement. Parts are stuck.
The corrosion of metal materials by salt fog is mainly caused by the electro-chemical reaction of conductive salt solution permeating the metal to form a “low-potential metal-electrolyte solution-high-potential impurity” microbattery system. Electron transfer occurs and the metal as the anode dissolves. New compounds are formed as corrosion products. The protective metal layer and the organic material protective layer are also the same. When the salt solution as an electrolyte penetrates into the interior, a micro-battery in which the metal is an electrode and the metal protective layer or the organic material is another electrode is formed.
Chloride ions play a major role in the salt fog corrosion damage process. It has a strong penetrating power, easily penetrates the metal oxide layer into the metal, and destroys the passive state of the metal. At the same time, chloride ions have very little hydration energy and are easily adsorbed on the surface of the metal, replacing the oxygen in the oxide layer of the protective metal and damaging the metal.
In addition to chloride ions, salt spray corrosion mechanisms are also affected by oxygen dissolved in the salt solution (substantially the salt solution that dissolves on the surface of the sample). Oxygen can cause the depolarization process of the metal surface and accelerate the dissolution of the anode metal. Since the salt spray process continues to spray, the salt liquid film continuously settles on the surface of the sample, so that the oxygen content is always kept near saturation. The formation of corrosion products expands the volume of the salt solution that penetrates into the metal defect, thereby increasing the internal stress of the metal, causing stress corrosion, and causing the protective layer to swell.
The main factors affecting the salt spray test results include: test temperature and humidity, salt solution concentration, sample placement angle, salt solution pH value, salt fog deposition amount and spray method.
Test temperature and humidity
Temperature and relative humidity affect the salt spray corrosion. The critical relative humidity for metal corrosion is about 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquesce to form an electrolyte with good conductivity. When the relative humidity decreases, the salt solution concentration will increase until the crystalline salt precipitates, and the corrosion rate will decrease accordingly.
The higher the test temperature, the faster the salt spray corrosion rate. The International Electrotechnical Commission's IEC 60355:1971 "AN APPRAISAL OF THE PROBLEMS OF ACCELERATED TESTING FOR ATMOSPHERIC CORROSION" standard states: "Each temperature rises by 10°C, the corrosion rate increases by 2 to 3 times, and the electrolyte conductivity increases by 10 to 20%." This is because of the increase in temperature, the increase in molecular motion, and the acceleration of chemical reactions. For the neutral salt spray test, most scholars believe that the test temperature is selected at 35 °C more appropriate. If the test temperature is too high, the salt spray corrosion mechanism differs from the actual situation.
Salt solution concentration
The effect of the salt solution concentration on the corrosion rate is related to the type of material and coating. When the concentration is less than 5%, the corrosion rate of steel, nickel and brass increases with the increase of concentration; when the concentration is greater than 5%, the corrosion rate of these metals decreases with the increase of the 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 salt concentration. In the low concentration range, the oxygen content increases with the salt concentration, but when the salt concentration increases to At 5%, the oxygen content is relatively saturated. If the salt concentration continues to increase, the oxygen content decreases accordingly. As the oxygen content decreases, the depolarizing ability of oxygen also decreases, which means the corrosion effect decreases. However, for zinc, cadmium, copper and other metals, the corrosion rate always increases with the concentration of salt solution.
Sample placement angle
The placement angle of the sample has a significant effect on the salt spray test results. The settlement direction of salt fog is close to the vertical direction. When the sample is placed horizontally, its projected area is the largest, and the amount of salt fog on the surface of the sample is also the most, so the corrosion is the most serious. The results show that when the steel plate and the horizontal line are at an angle of 45 degrees, the corrosion loss per square meter is 250 g. When the plane of the steel plate is parallel to the vertical line, the corrosion loss weight is 140 g per square meter. The GB/T 2423.17-93 standard stipulates that “the placement method of flat samples should be such that the test surface is at an angle of 30 degrees with the vertical direction.”
Salt solution pH
The pH of the salt solution is one of the main factors affecting the salt spray test results. The lower the pH, the higher the concentration of hydrogen ions in the solution, and the stronger the acidity and the stronger the corrosion. The salt spray test of Fe/Zn, Fe/Cd, Fe/Cu/Ni/Cr, etc. plating parts showed that the salt spray solution pH value 3.0 of the Acrylic Salt Spray Test (ASS) was more corrosive than the pH value of 6.5- The neutral salt spray test (NSS) of 7.2 is 1.5 to 2.0 times harsher.
Due to environmental factors, the pH of the salt solution changes. To this end, the salt spray test standards at home and abroad stipulated the pH range of the salt solution, and proposed a method for stabilizing the pH value of the salt solution during the test to improve the reproducibility of the salt spray test results.
Causes and results of changes in salt solution pH
1) The root cause of salt solution pH change during the salt spray test is mainly from the soluble substances in the air. The nature of these substances may be different. Some of them are acidic in water and some are alkaline in water.
2) During the salt spray test, the process of dissolving soluble substances in the air into the salt solution or escaping from the salt solution is a reversible process. The dissolved substance will cause the pH value of the salt solution to decrease, while the escaped substance will increase the pH of the salt solution. The rate of reduction and increase are equal, while the dissolution rate is greater than the escape rate, which will reduce the pH of the salt solution. . Conversely, the pH of the salt solution increases. If the dissolution and escape rates are equal, the pH does not change.
3) There are many factors that affect the pH change of salt solution. For example, the nature and content of soluble substances in air, pressure, contact area of air and salt solution, contact time, and the like.
a. The nature and content of soluble substances in the air
The air contains CO2, SO2, NO2, H2S, etc. These gases dissolve in water and produce acidic substances, which lowers the pH of the water. Alkaline dust particles may also be present in the air. These substances dissolve in water and increase the pH of the water.
b. Atmospheric pressure
The solubility of gas in water is proportional to atmospheric pressure. At 0°C, 0.355 g of CO2 can be dissolved in 100 ml of water at 1 atm of atmospheric pressure, while 0.670 g of CO2 can be dissolved in 100 ml of water at 2 atm of atmospheric pressure. When the compressed air is sprayed, since the atmospheric pressure increases, the dissolved amount of acidic substances such as CO2 in the air increases, and the pH of the salt solution decreases. This process is the opposite of the process in which CO2 is evolved from the salt solution after the temperature drops.
c. Contact area and contact time of air and salt solution
The spray turns the salt solution into a salt mist with fine particles of 1 to 5 μm in diameter. The increase in the contact area increases the amount of gas dissolved in the liquid or gas escaping from the liquid. When the conditions (such as pressure, temperature, etc.) that affect the evolution of gas into the liquid and the gas escape from the liquid are constant, the rate of dissolution and escape will eventually reach equilibrium. Before the equilibrium is reached, the amount dissolved (or escaped) will increase with time.
The results of the following three tests will show the effect of air and salt solution contact area and contact time on the pH value of the salt solution
The test results are shown in Table 1, Table 2, and Table 3.
1. Table 1: Change of storage time and pH value of saline solution in a 500 ml volumetric flask
Salt solution number Pre-storage pH Value Storage time pH after storage
I 7.2 88 days 7.2
II 7.2 88 days 7.1
Table 2: The effect of gas-liquid contact area and contact time on the pH of saline solution under general atmospheric conditions
Liquid container and diameter (mm) Stored in the atmosphere (hours)
0 4 10 24 168
Small bottle (Φ10) 7.0 7.0 7.0 7.0 7.0
Petri dishes (Φ100) 7.0 6.7 6.4 6.4 6.0
Table 3: Effect of Storage Conditions and Time on pH of Saline Solution in Alkaline-Containing Environment
Liquid container Salt solution storage time in alkaline cleaning workshop (days)
200ml bottle with lid 6.6 6.6 6.6 6.6 6.7 6.7 6.8 6.7
200ml capless bottle 6.6 6.9 7.2 7.3 7.5 7.7 7.7 7.7
24L without cover 6.5 - - - - 7.25 - -
Can be seen from Table 1, Table 2 and Table 3:
1 The pH of salt solution stored in a closed container does not change with the storage time. The reason is that there is no contact with air.
2 The salt solution stored in the Petri dish, with the increase of the gas-liquid contact time, its pH value decreased significantly. Obviously due to the larger contact area with air.
3 In an environment containing alkaline substances, the pH of the salt solution in the lidless container increases with the storage time.
The finer the salt spray particles are, the larger the surface area formed, the more oxygen is adsorbed, and the stronger the corrosion is. More than 90% of the salt spray particles in nature have a diameter of 1 micron or less. Research results show that the amount of oxygen adsorbed on the surface of salt spray particles with a diameter of 1 μm is relatively balanced with the amount of dissolved oxygen in the particles. Salt fog particles are smaller and the amount of oxygen that is adsorbed no longer increases.
Conventional spray methods include gas pressure spraying and spray tower methods. The most obvious drawbacks are the poor uniformity of salt spray settlement and the large diameter of salt spray particles. Ultrasonic atomization method uses the principle of ultrasonic atomization to atomize the salt solution directly into salt fog and enter the test area through diffusion. This solves the problem of poor uniformity of salt fog settlement, and the salt spray particle diameter is smaller. Different spray methods also have an effect on the pH of the salt solution (see Table 4).
Table 4: Effect of different spray methods on the pH change of saline solution
Spraying method pH value of spray salt solution Aggregation salt solution pH value
Pressure spray method 7.0 6.0 -1.0
Gas pressure spray tower method 7.4 6.5 -0.9
Ultrasonic atomization 7.0 6.9 -0.1
It can be seen from Table 4 that the ultrasonic atomization method without using compressed air has little effect on the pH of the salt solution, while the pH value of the salt solution changes significantly with the pneumatic spray method and spray tower method using the compressed air spray.
1) Principle of ultrasonic atomization
The principle of ultrasonic atomization is to use self-excited oscillations generated by the ultrasonic generator and transducer to radiate intense ultrasonic waves into the water. The ultrasonic waves transmit the salt solution to be atomized in the atomization cup through the water and the semi-permeable membrane, so that The microbubbles in the salt solution start to oscillate under the action of the sound field. When the sound pressure reaches a certain value, the microbubbles rapidly expand and close suddenly, and a shock wave is generated when the microbubbles are closed. This series of dynamic processes such as 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 surface of the liquid. The fine mist is driven by the flowing gas and the source continuously flows out of the atomization cup to achieve ultrasonic atomization. Only physical reactions occur throughout the process, and no chemical reactions occur.
Figure 1 Ultrasonic atomization device
2) Control of Salt Mist Settling in Ultrasonic Atomization
Ultrasonic atomization method can easily control the deposition rate of salt spray. The factors affecting the salt spray deposition rate include temperature, pressure, salt solution concentration, salt spray particle diameter, and atomization velocity. Salt spray particle size and ultrasonic frequency have the following relationship:
: ultrasonic frequency; : salt solution density; : surface tension of salt solution
It can be seen that when other conditions are certain, the salt spray deposition rate can be adjusted by adjusting the salt spray particle diameter. The higher the ultrasonic frequency, the finer the salt spray produced and the lower the salt fog sedimentation rate. The purpose of controlling the salt spray deposition rate can be achieved by adjusting the ultrasonic frequency.
The atomization speed is closely related to the power of the ultrasonic wave, and the salt spray deposition rate is adjusted by adjusting the power of the ultrasonic generator. So that the settling rate per unit time is controlled. The amount of salt spray can also be adjusted by adjusting the amount of air entering the atomizing cup inlet. When the intake air volume is large, micro-bubbles existing in the liquid will increase, and more fine mist will be easily formed. At the same time, the flow rate of the salt mist will increase due to the increase of the pressure difference, and the amount of mist entering the test area will increase.
In order to prove the feasibility and superiority of ultrasonic atomization, the following two tests will be conducted:
1 ultrasonic atomization feasibility test
The purpose of this test is 1) whether ultrasonically atomized salt fog has settled. (2) Can the salt spray settlement rate be controlled? (3) Whether the salt solution has been atomized has adverse physicochemical changes to the sample.
Figure 2 Ultrasonic atomization test Figure 3 Pressure spray test
The test structure is shown in Figure 2. The ultrasonic generator atomizes the salt solution in the atomizing cup and diffuses into the test area through the plastic hose. As the diffusion concentration increases, the salt mist begins to settle. The higher the concentration of salt fog in the test area, the faster the settlement. The final sedimentation 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 met the requirements of the salt spray standard.
2 salt spray settlement uniformity test
The purpose of this test is to demonstrate that the uniformity of salt spray settlement in ultrasonic atomization is significantly improved over the gas pressure spray method. Compared with the gas pressure spray method, the salt mist produced by the ultrasonic atomization method is fine and uniform, and its diameter can be controlled between several micrometers and 20 micrometers with good consistency. However, the salt spray particles produced by the gas pressure spray method are coarse and fine, and their diameters can reach several hundred microns. This results in uneven distribution of salt spray in the test area and reduces the effective test area.
1) Surface electroplating and electroless plating of metal substrates are performed under the "Manual Atmosphere Corrosion Test Salt Spray Test" GB/T10125-97
Artificial atmosphere corrosion test salt spray test method  :
a. Test solution
Chemically pure sodium chloride is dissolved in distilled or deionized water at a concentration of 50±5 g/L. Using a pH meter to measure the pH of the solution, you can also use a precision pH test paper calibrated with a pH meter for routine testing. The pH of the solution can be adjusted with chemically pure hydrochloric acid or sodium hydroxide. The pH value of the salt spray collection solution in the test chamber was 6.5-7.2. To avoid nozzle clogging, the solution must be filtered before use.
The type, number, shape, and size of the test specimens should be based on the requirements of the test overlay or product standard. If there is no standard, it can be decided in consultation with the relevant parties. Before the test, the sample must be fully cleaned. The cleaning method depends on the surface condition of the sample and the nature of the dirt. Do not use abrasives and solvents that will attack the surface of the specimen. After the sample is washed, contamination must be avoided. If the specimen is cut from the workpiece, the cover near the cutting area cannot be damaged. In addition to the regulations, the cutting area must be protected with a suitable covering, such as paint, sarcophagi, or adhesive tape.
c. Sample placement
The sample is placed in the test box with the test surface facing upwards, allowing the salt spray to settle freely on the test surface. The test surface cannot be sprayed directly by the salt spray. The angle at which the sample is placed is important. The test surface of the flat plate specimen is 15° to 30° from the vertical direction, and is 20° as far as possible. Samples with irregular surfaces (such as the entire workpiece) should also be as close as possible to the above requirements. Samples must not touch the cabinet or contact each other. The distance between the specimens should not affect the salt spray freely landing on the test surface. Drops on the specimen must not fall on other specimens. Sample holders are made of glass, plastic, and other materials. Suspended specimens shall not be made of metal. Man-made fibres, cotton fibres or other insulating materials shall be used. Drops on the holder must not fall on the specimen.
d. Test conditions
The temperature inside the spray box is 35±2°C. After the salt mist settles, after 24 h spraying, each collected solution should be 1-2 ml/h for 80 cm, 50±10 g/L sodium chloride, and pH 6.5-7.2. Through the mist in the sample area, no more use
e. Test cycle
The time of the test should be determined according to the requirements of the test coverage or product standard; if there is no standard, it can be decided through consultation of relevant parties. The recommended test time is: 2, 6, 16, 24, 48, 96, 240, 480, 720h. The spray must not be interrupted during the specified test period. The salt spray box can only be opened when the sample is to be briefly observed. If the end point of the test depends on the time at which corrosion begins, the sample needs to be checked frequently. Therefore, these specimens cannot be tested together with specimens having a predetermined test period. The test for a predetermined period can be checked in the above cycle. However, the test surface cannot be destroyed during the inspection process. The time for the inspection of the specimens should be as short as possible.
f. Cleaning of test specimen after test
At the end of the test, remove the sample. In order to reduce the loss of corrosion products, the sample was naturally dried in the room for 0.5-1 h before cleaning. Then gently rinse with clean running water no higher than 40°C to remove the residue from the salt solution on the surface of the sample and immediately blow dry with a hair dryer.
g. Evaluation of test results
The test results are compared with the test results of specimens and specimens after corrosion test of metal and other inorganic coatings on metal substrates by the GB/T6461-2002 technical standards and the agreement between the two parties, and they are qualified within the scope of the standard, and vice versa. Failed to pass the test.
2) Anodized parts of aluminum and aluminum alloys, using copper-accelerated acetate spray test (CASS) standard for oxide films
Copper Accelerator Acetate Spray Testing (CASS) Method for Anodic Oxidation of Aluminum and Aluminum Alloys  :
a. Test solution
The analytically pure sodium chloride was dissolved in distilled or deionized water to a concentration of 50±5 g/L. In this sodium chloride solution, analytically pure copper dichloride (CuCl2•2H2O) was added to a concentration of 0.26±0.02 g/L (0.205±0.015 g/L CuCl2). The pH of the solution was adjusted to 3.0-3.1 with analytically pure glacial acetic acid and sodium hydroxide. The PH value should be measured with a pH meter at 25°C or daily inspection with precision pH test paper. The solution must be filtered before use to avoid clogging the nozzle.
b. Specimen (same as Metal Cover Neutral Salt Spray Test (NSS) Standard b)
c. Specimen placement (same as Metal Cover Neutral Salt Spray Test (NSS) Standard c)
d. Test conditions
The temperature inside the spray box is 35±2°C. After the salt mist has settled, after 24 hours of spraying, each collected solution should be 1-2 ml/h for 80 cm, 50±10 g/L for sodium chloride, and 3.0-3.1 for PH. The mist that passed through the sample area must not be used again. In order to compare test conditions in different laboratories or on different days, nickel plates can be used for calibration.
e. Test cycle
The time of the test should be determined according to the requirements of the test coverage or product standard; if there is no standard, it can be decided through consultation of relevant parties. The recommended test times are: 4, 8, 16, 26, 32, 40, 48, 56, 64, 72h. The spray must not be interrupted during the specified test period. The salt spray box can only be opened when the sample is to be briefly observed.
f. Cleaning of test specimen after test
At the end of the test, remove the sample. Dry naturally for 0.5-1 h, then gently rinse with clean flowing water no higher than 40°C to remove the residue from the salt spray solution on the surface of the sample, and immediately dry the sample with compressed air or a hair dryer not exceeding 200 kPa.
g. Evaluation of test results (with metal coating neutral salt spray test (NSS) standard)
3) Test results and criteria
The measured test results are compared with the technical standards of the relevant products and the agreement between the two parties. The standards are judged to be acceptable within the scope of the standard. Otherwise, the tests are rejected. Unless otherwise specified, conventional records need only consider the following aspects:
a. The appearance after the test;
b. The appearance after removing the corrosion product;
c. The distribution and quantity of corrosion defects such as pitting, cracks, bubbles, etc.;
The above test results were evaluated according to the methods specified in GB/T 6461-2002 "Ranks and Specimens of Metals and Other Inorganic Coatings on Metal Substrates After Corrosion Test."
The sample for the finished test shall be placed in the sample bag, and the condition of the sample and the test date shall be written on the sample bag. Generally, the sample shall be kept for more than six months.
This method is based on the appearance characteristics of the corrosion products after the salt spray test. The corrosion characteristics of common plating parts after the salt spray test are shown in Table 5 below.
Table 5: Corrosion characteristics of common plating parts after salt spray test
Plating Parts Type Corrosion Characteristics
Steel galvanized Gray or black plating corrosion and brown rust
Cadmium plated cadmium gray or black coating corrosion and brown rust
Steel chrome plated Brown rust
Copper plated green
Copper tin plating Gray plating corrosion and green copper rust
The standards adopted by this method are: JB4159-1999 "General Technical Requirements for Tropical Electrical Products"; GJB4.11-1983 "Salt Spray Test for Marine Electronic Equipment Environmental Test"; GB/T4288-2003 "Home Electric Washing Machine" etc.
Percentage of corrosion
This method is suitable for flat samples. If the test time is short or the sample shape is complex, the corrosion area is difficult to measure.
The standards adopted by this method are: GB/T6461-2002 "Ranks and Specimen Ratings of Metals and Other Inorganic Coatings on Metal Substrates After Corrosion Test".
The calculation formula of GB/T6461-2002:
Where: A: the percentage of the total area covered by corrosion; R: the protection level (see Table 6), which is divided into 0 to 10 levels.
Table 6: R-protection level table
A level A level
No defect 10 2.5<A≤5 4
A≤0.1 9 5<A≤10 3
0.1<A≤0.25 8 10<A≤25 2
0.25<A≤0.50 7 25<A≤50 1
0.50<A≤1.00 6 A>50 0
Standards that employ this method include ASTM B537-1970 "Standard Practice for Rating of Electroplated Panels Subjected to Atmospheric Exposure" and the like.
This method uses 5 × 5 (mm) as a small square, the main surface of the sample is divided into a number of small squares, calculate the corrosion rate of the sample, see Table 7 for the classification of corrosion rate.
Table 7: Corrosion Rate Rating Table
Corrosion rate % grade Corrosion rate % grade
0 10 ≤ 8 4
≤0.25 9 ≤16 3
≤0.5 8 ≤32 2
≤1 7 ≤64 1
≤2 6 >64 0
≤ 4 5
Increase or decrease by weight
This method is based on the weight of the sample caused by the corrosion of the material, weighing the sample before and after the weight change, divided into weight loss method and weight gain method. Both of these methods are usually plate-like samples.
Weight loss method is to use a chemical solvent that can dissolve corrosive substances and can't react chemically to the sample itself, dissolve the corrosive substances on the sample after the test, and make the weight of the sample after the test lighter than that before the test. The weight loss method is expressed as the value of the weight loss of the unit sample area after the test.
The weight gain method directly measures the value of the increase in weight per unit area after the test.
Divided by experience
This method is based on the actual work experience to divide the corrosion degree of the sample after the salt spray test, which is a very rough representation method. The following statements are commonly used: very corrosive, severely corrosive, moderately corrosive, slightly corrosive, very slightly corrosive, good appearance, etc.
The salt spray test is an important means for assessing the ability of products or materials to resist salt spray corrosion. The scientificness and rationality of the test results are crucial. There are many factors that affect the stability and consistency of salt spray test results. To improve the effectiveness of the salt spray test results, the test technology is the key. Therefore, testers not only need to have solid professional knowledge and professional skills, but also need rich practical experience and comprehensive understanding of the product. They must understand the salt spray test from multiple disciplines such as chemistry and environmental engineering, materials, structures and processes, and be scientific and rational. The test results are expressed to provide better information for product selection, structural design, process selection, product transportation, storage and use, and to improve the salt spray corrosion resistance of the product or material.