The salt spray test (or salt mist) is a standard and popular corrosion test method, used to check corrosion resistance of materials and surface coating. Typically, the material to be tested is metal (although stone, ceramic, and polymer can also be tested) and finished with a surface coating intended to provide a degree of corrosion protection on the underlying metal. Salt spray testing is an accelerated corrosion test that results in corrosive attacks for coated samples to evaluate (mostly comparably) the suitability of the coating to be used as a protective coating. The appearance of corrosion products (rust or other oxides) is evaluated after a predetermined period of time. Duration of the test depends on the corrosion resistance of the coating; generally, the more resistant corrosion layer, the longer the test period before the appearance of corrosion/rust. The salt spray test is one of the most extensive and long corrosion tests. ASTM B117 is the first internationally recognized salt spray standard, published in 1939. Other important relevant standards are ISO9227, JIS Z 2371 and ASTM G85.
Video Salt spray test
Apps
Salt spray test is very popular because it is relatively cheap, fast, well standardized, and can be repeated fairly. Although there may be a weak correlation between duration in salt spray tests and the expected life of layers in certain coatings such as galvanized steel, these tests have gained popularity worldwide due to low cost and rapid results. Most Salt Spray Spaces are currently used NOT to predict the corrosion resistance of layers, but to maintain coating processes such as pre-treatment and painting, electroplating, galvanizing, and the like, comparatively. For example, a pre-treated paint component must pass 96 hours of Neutral Salt Spray, in order to be accepted for production. Failure to meet these requirements implies instability in the pre-treatment chemical process, or the quality of the paint, which must be addressed immediately, so that future batches have the desired quality. The longer the corrosion test is accelerated, the longer the process is uncontrolled, and the greater the loss in the form of unsuitable batches. Therefore, the application of the salt spray test principle allows a quick comparison made between actual and expected corrosion resistance. Most commonly, the time required for the oxide to appear in the tested sample is compared with expectations, to determine whether the test is passed or failed. For this reason salt spray tests are most commonly used in quality audit roles, where, for example, can be used to check the effectiveness of a production process, such as coating the surface of metal parts. The salt spray test has little application in predicting how the surface material or coating will resist corrosion in the real world, as it does not create, replicate or accelerate real world corrosive conditions. The cyclic corrosion test is better suited for this.
Maps Salt spray test
Testing equipment
The testing apparatus comprises a closed cabinet/test chamber, in which a brine solution (5% NaCl) is atomized by using a spray nozzle using pressurized air. This results in a thick corrosive environment of heavy brine (also referred to as mist or spray) in the room, so that the test samples exposed to this environment experience severe corrosive conditions. Room volume varies from supplier to supplier. If there is a minimum volume required by a specified salt spray test standard, this will be clearly stated and must be met. There is a general historical consensus that larger rooms can provide a more homogeneous testing environment.
The variation of the salt spray test solution depends on the material to be tested. The most common test for steel-based materials is the Neutral Salt Spray test (often abbreviated as NSS) reflecting the fact that this test solution type is prepared for a neutral pH of 6.5-7.2. Results are represented in general as testing hours in NSS without the appearance of corrosion products (eg 720 hours in NSS according to ISO 9227). A synthetic sea water solution is also commonly determined by several companies and standards. Other test solutions contain other chemicals added including acetic acid (often abbreviated ASS) and acetic acid with copper chloride (often abbreviated CASS) each selected for the evaluation of decorative coatings, such as copper-nickel-chromium, copper plated or aluminum anodized. This acidified test solution generally has a pH of 3.1-3.3
Some sources do not recommend using ASS or CASS test cabinets alternately for NSS tests, because of the risk of cross contamination, it is claimed that thorough cabinet cleaning after a CASS test is very difficult. ASTM does not address this issue, but ISO 9227 does not recommend it and if it should be done, the proponents of the cleanup are thorough.
Although most salt spray tests are continuous, ie; the tested sample was exposed to salt formation continuously for the entire duration of the test, some did not require such exposure. This kind of test is usually referred to as a modified salt spray test. ASTM G85 is an example of a test standard that contains some modified salt spray test which is a variation of the basic salt spray test.
Modified Salt Spray Test
ASTM G85 is the most popular global test standard that includes a modified salt spray test. There are five such tests, all of which are mentioned in ASTM G85 as attachments A1 through A5.
Many of these modified tests originally appeared in a particular industry sector, to address the need for corrosion testing that was able to replicate the corrosion effects that occur naturally and accelerate this effect.
This acceleration arises through the use of a chemically altered salt spray solution, often combined with other test temperatures and in many cases, the relatively fast cycle of this test climate over time. Although popular in certain industries, modification of salt spray testing has in many cases been replaced by cyclic corrosion testing (CCT). The type of environmental test chamber used for modification of salt spray testing for ASTM G85 is generally similar to those used for ASTM B117 testing, but will often has some additional features, such as automatic climate cycling control system.
ASTM G85 attachment A1 - Acetic acid salt test (non-cyclic) This test can be used to determine the relative resistance to the corrosion of decorative chrome plating on die casting of steel and zinc when exposed to acetic acid spray salt climates at temperature high. This test is also referred to as the ASS test. The test specimens were placed in a confined space and exposed continuously sprayed continuous aqueous solutions, prepared in accordance with standard test requirements and acidified (up to pH 3.1-3.3) with the addition of acetic acid. This spray is set to fall to the specimen at a rate of 1.0 to 2.0 ml/80Ã, cmÃ,²/hr, in room temperature 35C. This climate is maintained in steady state conditions constant. Test duration varies.
ASTM G85 attachment A2 - Associated Fogs Smoke Test (cyclic).
This test can be used to test the relative resistance to corrosion of aluminum alloys when exposed to climatic changes of acetic acid salt spray, followed by air drying, followed by high humidity, all at high temperatures. This test is also referred to as the MASTMAASIS test. The test specimen is placed in a confined space, and subjected to climate change consisting of 3 parts of the following recurring cycle. Exposure 0.75 hours to continuous indirect salt water spray, prepared in accordance with the standard test requirements and acidified (up to pH 2.8 to 3.0) with the addition of acetic acid. This spray is set to fall to the specimen at a rate of 1.0 to 2.0ml/80Ã, cmÃ,²/hour. This is followed by 2.0 hours of exposure to air drying (cleaning) of climate. This was followed by 3.25 hours of exposure to high humidity climates that gradually increased to between 65% RH and 95% RH. The entire test cycle is at a constant temperature of 49C. The number of cycles repeated and therefore the duration of the test varies.
ASTM G85 attachment A3 - Asserted (cyclic) Sea Water Test
This test can be used to test the relative resistance to corrosion of coated or non-coated aluminum alloys and other metals, when exposed to atmospheric acid spray synthetic spray change, followed by high humidity, both at high temperatures. This test is also referred to as SWAAT test. The test specimens are placed in a confined space, and subject to climate change consisting of two cycles of repetition of the following sections. The 30 minute exposure to the indirect continuous spray of a synthetic seawater solution, prepared in accordance with the standard test requirements and acidified (up to pH 2.8 to 3.0) with the addition of acetic acid. This spray is set to fall to the specimen at a rate of 1.0 to 2.0ml/80Ã, cmÃ,²/hour. This was followed by 90 minutes of exposure to a high humidity climate above 98% RH. The entire test cycle is at a constant room temperature of 49C (can be reduced to 24 to 35C for organic coated specimens). The number of cycles repeated and therefore the duration of the test varies.
ASTM G85 lampiran A4 - SO2 Salt Spray Test (siklik)
This test can be used to test the relative resilience to product sample corrosion which is likely to confront the combined SO2 (sulfur dioxide)/spray salt/acid rain environment during their normal service life. The test specimens were placed in a confined space, and exposed to 1 of 2 possible cycles of climate change. In both cases, salt spray exposure may be a brine or synthetic sea water spray prepared in accordance with the standard test requirements. The most appropriate test cycles and spray solutions should be agreed between the parties.
The first climatic cycle consisted of a continuous indirect spray of neutral (pH 6.5-7.2) of salt water/a synthetic seawater solution, which fell to the specimen at a rate of 1.0 to 2.0 ml/80 cm/2 h. During this spraying, the room was given SO2 gas with a volume rate of 35Ã, cm³³/min/mà ,,, for 1 hour every 6 hours of spraying. The entire test cycle is at a constant space temperature of 35C. The number of cycles repeated and therefore the duration of the test varies.
The second climatic cycle consists of 0.5 hours of indirect continuous spray from neutral (pH 6.5-7.2) of salt water/synthetic seawater solution, which falls to the specimen at a rate of 1.0 to 2.0 ml/80 cm/2 hours. This is followed by 0.5 hour dose with SO2 gas at volume level 35Ã, cm³³/min/mÃ,³. This is followed by a 2.0 hour high moisture soak. The entire test cycle is at a constant space temperature of 35C. The number of cycles repeated and therefore the duration of the test varies.
ASTM G85 attachment A5 - Salt/Dry Test Electrolyte (cyclic) electrolyte
This test can be used to test the relative resilience to corrosion paint on steel, when exposed to a changing climate of aqueous salt spray at room temperature, followed by air drying at high temperatures. This is a popular test in the surface coating industry, where it is also referred to as the PROHESION (TM) test. The test specimens are placed in a confined space, and subject to climate change consisting of two cycles of repetition of the following sections. 1.0 hours exposure to continuous indirect saline water spray, prepared in accordance with the standard test requirements and acidified (up to pH 3.1-3.3) with the addition of acetic acid. This spray is set to fall to the specimen at a rate of 1.0 to 2.0 ml/80Ã, cmÃ,²/hour, in ambient room temperature (21 to 27C). This is followed by exposure of 1.0 hours for air drying (cleaning) of climate, in room temperature 35C. The number of cycles repeated and therefore the duration of the test varies.
Standardization
Room construction, testing procedures and testing parameters are standardized under national and international standards, such as ASTM BÃ, 117 and ISO 9227. These standards describe the information required to carry out this test; test parameters such as temperature, air pressure from sprayed solution, preparation of spray solution, concentration, pH, etc. Daily checking of test parameters is required to demonstrate compliance with the standards, so records should be maintained accordingly. ASTM B117 and ISO 9227 are widely used as reference standards. The test cabinets are manufactured in accordance with the requirements specified herein.
However, this testing standard does not provide information about the testing period for the coating to be evaluated, or the appearance of corrosion products in salt form. Requirements agreed between the customer and the manufacturer. In the automotive industry requirements are determined on the basis of material specifications. Different layers have different behaviors in the salt spray test and consequently, the duration of the test will vary from one type of layer to another. For example, a typical passive metal and passive metal zinc section lasts for 96 hours in a non-rust-white salt spray test. The nickel-nickel-steel parts that are adapted can last more than 720 hours in a non-rust-free NSS test (or 48 hours in a red-rust-free CASS test) The requirements are specified in the test duration (hours) and the coating should be in accordance with the minimum testing period.
Artificial sea water sometimes used for Salt Spray Testing can be found at ASTM International. The Standard for Artificial Sea Water is ASTM D1141-98 which is a standard practice for the preparation of replacement seawater.
Usage
Special coatings that can be evaluated by this method are:
- Pigmented surface (pre-treated) (with subsequent paint/primer/lacquer/rust prevention)
- Zinc and zinc-alloy plating (see also electroplating). See ISO 4042 for guidance
- Chromium that is adapted, nickel, copper, tin
- Coatings are not applied electrolytically, such as zinc coatings according to ISO 10683
- Organic coatings, such as rust prevention
- Cat Coating
Hot-dip galvanized surfaces are generally not tested in salt spray test (see ISO 1461 or ISO 10684). Hot-dip galvanizing produces zinc carbonate when exposed to the natural environment, thus protecting the coating metal and reducing the corrosion rate. Zinc carbonate is not produced when a galvanized specimen is exposed to a salt spray mist, therefore this test method does not provide an accurate measurement of corrosion protection. ISO 9223 provides guidance for accurate corrosion resistance measurement for hot-dip galvanized specimens.
The painted surface with the underlying hot-dip galvanized coating can be tested according to this method. See ISO 12944-6.
The test period ranges from a few hours (eg 8 or 24 hours of phosphate steel) to more than a month (eg 720 hours of zinc-coating, 1000 hours of specified zinc flake coating).
Bibliography
- Metal Finishing. Guidebooks and directory problems. Published by Metal Finishing Magazine, 1996
See also
- Cyclic corrosion test
- Environmental space
- Japanese Industrial Standard
- ASTM International
- International Organization for Standardization
Further reading
- ASTM G85 Modified Standard Spray Salt Test
- Deutsches Institut fÃÆ'¼r Normung e.V. DIN 50021 SprÃÆ'¼hnebelprÃÆ'¼fungen mit verschiedenen NatriumchloridlÃÆ'¶sungen. Beuth Verlag GmbH, 1988. This standard has been supplanted by ISO 9227 and is only mentioned for bibliographic purposes
- ISO International Organization for Standardization. ISO 9227 Corrosion test in artificial atmosphere - Salt salt test, 2006
- ISO International Organization for Standardization. ISO 4628-3 Paint and varnish. Evaluate coating degradation. Determination of the quantity and size of defects, and the intensity of uniform change in appearance. Section 3 Raw rate assessment
- MIL-STD-810 Environmental Engineering Considerations and Laboratory Tests
- ASTM B117 Terms, Methods & amp; Application
References
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