This produces a corrosive environment of dense salt water fog also referred to as a mist or spray in the chamber, so that test samples exposed to this environment are subjected to severely corrosive conditions. Chamber volumes vary from supplier to supplier. If there is a minimum volume required by a particular salt spray test standard, this will be clearly stated and should be complied with. There is a general historical consensus that larger chambers can provide a more homogeneous testing environment.
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This produces a corrosive environment of dense salt water fog also referred to as a mist or spray in the chamber, so that test samples exposed to this environment are subjected to severely corrosive conditions. Chamber volumes vary from supplier to supplier.
If there is a minimum volume required by a particular salt spray test standard, this will be clearly stated and should be complied with. There is a general historical consensus that larger chambers can provide a more homogeneous testing environment. Variations to the salt spray test solutions depend upon the materials to be tested. The most common test for steel based materials is the Neutral Salt Spray test often abbreviated to NSS which reflects the fact that this type of test solution is prepared to a neutral pH of 6.
Results are represented generally as testing hours in NSS without appearance of corrosion products e. Synthetic seawater solutions are also commonly specified by some companies and standards. Other test solutions have other chemicals added including acetic acid often abbreviated to ASS and acetic acid with copper chloride often abbreviated to CASS each one chosen for the evaluation of decorative coatings, such as electroplated copper-nickel-chromium, electroplated copper-nickel or anodized aluminum.
These acidified test solutions generally have a pH of 3. It is claimed that a thorough cleaning of the cabinet after CASS test is very difficult. ASTM does not address this issue, but ISO does not recommend it and if it is to be done, advocates a thorough cleaning. Although the majority of salt spray tests are continuous, i. Such tests are commonly referred to as modified salt spray tests. ASTM G85 is an example of a test standard which contains several modified salt spray tests which are variations to the basic salt spray test.
A modified salt spray chamber in use  Many of these modified tests originally arose within particular industry sector , in order to address the need for a corrosion test capable of replicating the effects of naturally occurring corrosion and accelerate these effects. This acceleration arises through the use of chemically altered salt spray solutions, often combined with other test climates and in most cases, the relatively rapid cycling of these test climates over time.
Although popular in certain industries, modified salt spray testing has in many cases been superseded by cyclic corrosion testing CCT The type of environmental test chambers used for modified salt spray testing to ASTM G85 are generally similar to the chambers used for testing to ASTM B , but will often have some additional features, such as an automatic climate cycling control system. This test is also referred to as an ASS test. Test specimens are placed in an enclosed chamber and exposed to a continuous indirect spray of salt water solution, prepared in accordance with the requirements of the test standard and acidified pH 3.
This climate is maintained under constant steady state conditions. The test duration is variable. This test can be used to test the relative resistance to corrosion of aluminium alloys when exposed to a changing climate of acetic acid salt spray, followed by air drying, followed by high humidity, all at an elevated temperature. Test specimens are placed in an enclosed chamber, and exposed to a changing climate that comprises the following 3 part repeating cycle. This is followed by a 2 hour exposure to an air drying purge climate.
This is followed by 3. The number of cycle repeats and therefore the test duration is variable. Test specimens are placed in an enclosed chamber, and exposed to a changing climate that comprises the following 2 part repeating cycle. First, a 30 minute exposure to a continuous indirect spray of synthetic seawater solution, prepared in accordance with the requirements of the test standard and acidified pH 2.
Test specimens are placed in an enclosed chamber, and exposed to 1 of 2 possible changing climate cycles. In either case, the exposure to salt spray may be salt water spray or synthetic sea water prepared in accordance with the requirements of the test standard. The most appropriate test cycle and spray solutions are to be agreed between parties. The first climate cycle comprises a continuous indirect spray of neutral pH 6. The second climate cycle comprises 0.
This is followed by 0. This is followed by 2 hours of high humidity soak. Test specimens are placed in an enclosed chamber, and exposed to a changing climate with the following 2-part cycle. First, a 1-hour exposure to a continuous indirect spray of salt water solution, prepared in accordance with the requirements of the test standard and acidified pH 3. The cycle repeats until the desired duration has been achieved. These standards describe the necessary information to carry out this test; testing parameters such as temperature, air pressure of the sprayed solution, preparation of the spraying solution, concentration, pH , etc.
Daily checking of testing parameters is necessary to show compliance with the standards, so records shall be maintained accordingly.
Testing cabinets are manufactured according to the specified requirements here. However, these testing standards neither provide information of testing periods for the coatings to be evaluated, nor the appearance of corrosion products in form of salts.
Requirements are agreed between customer and manufacturer. In the automotive industry requirements are specified under material specifications. Different coatings have different behavior in salt spray test and consequently, test duration will differ from one type of coating to another.
For example, a typical electroplated zinc and yellow passivated steel part lasts 96 hours in salt spray test without white rust. Electroplated zinc-nickel steel parts can last more than hours in NSS test without red rust or 48 hours in CASS test without red rust Requirements are established in test duration hours and coatings shall comply with minimum testing periods. Hot-dip galvanizing produces zinc carbonates when exposed to a natural environment, thus protecting the coating metal and reducing the corrosion rate.
The zinc carbonates are not produced when a hot-dip galvanized specimen is exposed to a salt spray fog, therefore this testing method does not give an accurate measurement of corrosion protection. ISO gives the guidelines for proper measurement of corrosion resistance for hot-dip galvanized specimens. Painted surfaces with an underlying hot-dip galvanized coating can be tested according to this method. See ISO Testing periods range from a few hours e.
Section 3. Number of test specimens To be specified, typically 1 Shape and size of specimens To be specified, typically 75 x mm Arrangement of the test specimens The specimen must be flat and placed in the cabinet facing upwards at an angle as close as possible to 20 o to the vertical. This angle shall, in all cases, be within the limits 15 o to 25 o. Duration of tests To be specified. ISO evaluation of results Various criteria may be applied to meet particular requirements: Appearance after the test; Appearance after removing superficial corrosion products; Number and distribution or corrosion defects, i.
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It also describes the method employed to evaluate the corrosivity of the test cabinet environment. It does not specify the dimensions or types of test specimens, the exposure period to be used for a particular product, or the interpretation of results. Such details are provided in the appropriate product specifications. The salt spray tests are particularly useful for detecting discontinuities, such as pores and other defects, in certain metallic, organic, anodic oxide and conversion coatings. The neutral salt spray NSS test particularly applies to - metals and their alloys, - metallic coatings anodic and cathodic , - conversion coatings, - anodic oxide coatings, and - organic coatings on metallic materials. It has also been found suitable for testing anodic and organic coatings on aluminium. The salt spray methods are all suitable for checking that the quality of a metallic material, with or without corrosion protection, is maintained.
Equivalence between ASTM B-117 vs. ISO 9227 NSS
The test duration is variable. The salt solution should be prepared according to the nature of the test in accordance with Paragraph 3 of ISO and the 3 options are summarised below; NSS Neutral Salt Spray - The pH of the salt solution fall-out should be maintained at a neutral pH pH 6. The salt fog fall-out rate is verified by using a minimum of two 80cm2 funnels per salt spray atomiser, each inserted into a measuring cylinder graduated in ml. These collectors are located inside the chamber, nominally at the same height as the test samples, one located close to the atomiser and one located away from the atomiser, in accordance with the positions specified in ASTM B, but also taking into account the chamber manufacturers defined work-space. Typical Application This test can be used to test the relative resistance to corrosion of coated and uncoated metallic specimens, when exposed to a salt spray climate at an elevated temperature. Test duration depends on the corrosion resistance of the coating; generally, the more corrosion resistant the coating is, the longer the period of testing before the appearance of oxides.