Salt spray test

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Salt spray test (also known as “salt spray test”) is a standardized test for evaluating the corrosion protection effect of organic coatings, metallic coatings or chemical or physical surface treatments. Various national or international standards (e.g. ASTM B117, DIN EN ISO 9227) regulate the performance of the test. The test pieces are positioned under standardized conditions in a test chamber in which a sprayed salt solution (usually a solution of sodium chloride ) acts on the test pieces. The test is often carried out continuously until a sufficient differentiation between the test specimens has been achieved or a predetermined test duration has been reached. This can be a few hours to a few thousand hours. At the end of the test period, the signs of corrosion on the test specimens are assessed, e.g. B. according to the degree of rust, degree of bubbles and corrosive infiltration of organic coatings. A connection between the corrosion that occurred in the salt spray test and the corrosion to be expected under real conditions is rather rare, as the corrosion mechanisms usually differ. The ASTM Committee G-1 for the Corrosion of Metals, responsible for the ASTM B 117 salt spray standard, has therefore drafted the following resolution for the application of ASTM B 117: "The ASTM Committee G-1 for the Corrosion of Metals confirms that results of salt spray tests according to ASTM B 117 rarely match behavior in natural environments. "

The salt spray test is used as a test method for the quality control of coatings in industry.

principle

The principle of the salt spray test is simple: In the chamber, the sprayed salt solution produces a corrosive atmosphere, which causes a corrosion attack on the exposed parts. Under these conditions the corrosion process accelerates and the coatings lose their corrosion protection during the test. The parts corrode faster than normal conditions in the application, e.g. B. in the car, in the construction parts, in the buildings etc. The duration of the test depends on the requirements of the coating. Since the concentration of the aqueous salt solution, temperature, pressure and pH value must be kept constant, the results can be reproduced.

Test facility

The chamber should be of a usable size. Larger capacities are available on request, as large parts (e.g. screws for wind turbines) cannot be tested in a small chamber. The construction material of the inner housing of the chamber, the frames, must be made of inert and corrosion-resistant materials so that no corrosive reaction occurs between the sample to be tested and the material of the chamber. There are one or more nozzles in the chamber that spray the solution. Larger chambers need more than one nozzle for correct distribution of the mist.

The chamber must be equipped with adjustable compressed air so that the corrosive mist is sprayed through the nozzle. There is a separate container with the solution that has to be sprayed during the test. The chamber must also be equipped with a heating system so that the correct temperature of the chamber can be maintained during the test.

execution

The parts must be hung with the smallest possible contact points or mounted in a frame. The parts must be distributed in such a way that no condensate can drip onto the parts positioned below and no contact between parts is possible. Sheets and other parts (e.g. screws) must be positioned at an angle (approx. 20 °) from the vertical.
The following parameters must be observed in the salt spray test with NaCl solution in accordance with DIN EN ISO 9227: pH value 6.5 to 7.2; Concentration of the NaCl solution 50 ± 5 g / l, temperature of the chamber 35 ° C ± 2 ° C, overpressure at the nebulization nozzle 0.7 to 1.4 bar and the condensed volume of the sprayed solution, which is filled with a funnel Ø 100 mm in a calibrated measuring cylinder is collected, 1 to 2 ml per hour.

There are some deviations in the parameters of the ASTM B117, ISO 9227 standards and also to the in-house standards of the automotive industry, in principle all parameters are similar. It is necessary to guarantee that the correct parameters of the test are maintained within the specified tolerances; a log is necessary. In addition to the NaCl solution, other solutions can be used; the variants are abbreviated as follows:

  • Sodium chloride solution with neutral pH value: NSS test ( neutral salt spray = neutral salt spray test)
  • Acetic acid to the normal NaCl solution: AASS test ( acetic acid salt spray = acetic acid salt spray test)
  • Test under a marine climate with the addition of CaCl 2 and baking soda to the normal NaCl solution according to OEM standards
  • Copper (II) chloride dihydrate for normal NaCl solution: CASS test ( copper accelerated salt spray test )

ASS and CASS tests are suitable for decorative galvanic layers such as Cu-Ni-Cr or Ni-Cr coatings and also anodic layers for aluminum. It is very difficult to clean a chamber that has been used for ASS or CASS tests. Therefore, NSS test chambers are only recommended for NSS tests.

standardization

The international standards such as ASTM B117 and DIN EN ISO 9227, which specify the performance of this test, do not prescribe any method for evaluating the corroded surface or testing times for the samples; the assessment after the appearance of the corrosion products must be agreed between the customer and the manufacturer. In the automotive industry there are standards for evaluating the quality of coated surfaces. Coated steel produces red rust (iron oxide) after the corrosion attack, when the coating no longer offers protection; a zinc layer creates zinc oxide after a few hours in the test chamber. Zinc oxide is known as white rust . A coating system must provide a minimum test duration without corrosion in order to meet the specified requirements. A coated sample with pure zinc produces white rust during the corrosion process after a few hours in the test chamber. This corrosion attack occurs as corrosion points that can be observed on the surface as white points. The corrosion process develops until red spots appear on the surface. Corrosion of the iron (base metal) is achieved. In order to protect the zinc layer against corrosion, passivations, chromating, powder coatings, etc. can be applied. These layers inhibit the occurrence of the zinc corrosion products and therefore white rust is not seen that early in the NSS test.

The in-house standards of the automotive industry and other branches of industry refer to these reference standards. Each coating system offers special corrosion properties and the customer's delivery specifications define specific requirements with regard to the corrosion protection of the surface. The materials to be tested must withstand a given test duration without corrosion products in order to meet the requirements of the standard.

criticism

The salt spray test (NSS) was developed to evaluate the protective effect of organic coatings. The operating principle is that of a weak point or pore test and uses a substrate that reacts with the test medium (water, common salt), e.g. B. a base metal ahead. During the test, the test medium penetrates the organic coating and - as soon as the metallic substrate is reached - leads to the formation of corrosion products. In sufficient quantities, these can be seen in the form of bubbles in the coating. In this way, chemically inactive organic coatings on the same substrate can be compared with one another in terms of their protective effect.

However, the salt spray test is not suitable for comparing chemically active organic coatings, metallic coatings or base metals with different reactivity with one another. Similar organic coatings on steel and zinc will generally show different protective effects in the NSS. Also, there is only extremely seldom a correlation between the test results from the salt spray test and corrosion processes under natural weathering, because the NSS represents a permanently humid atmosphere that is rarely found in reality. These circumstances were explicitly taken into account in DIN EN ISO 9227 Corrosion tests in artificial atmospheres - salt spray tests: “ There is only rarely a direct connection between resistance to the effects of salt spray and resistance to corrosion in other media. The various factors that influence the progression of corrosion can have very different effects depending on the prevailing conditions. This includes B. also the formation of protective layers. The test results should therefore not be viewed as a direct indication of the corrosion resistance of the metallic materials tested in all environmental conditions in which these materials can be used. ”In addition, the standard indicates that the test is only suitable for quality control.

Many specialist articles checked by experts expressly warn against the use of the salt spray test. Here are some examples:

“The well-known ASTM B-117 salt spray test compares cold-rolled and hot-dip galvanized steel and delivers results within a few hours. Unfortunately, the test does not manage to demonstrate the proven higher corrosion resistance of the hot-dip galvanized sample compared to the non-galvanized sample. "

“The salt spray test is the most frequently used test for accelerated corrosion testing. It was developed over 50 years ago to check the corrosion behavior of metal coatings in a maritime environment. Although it has been shown many times that this test does not give a reliable indication of the corrosion resistance of coatings outdoors (not even in a salty atmosphere), it has become firmly established in the coating industry ”.

“Spraying with salt ensures rapid degradation of the surface. However, this can hardly be compared with the deterioration in the material properties under real conditions. The degradation triggered by salt follows different mechanisms than the degradation under real external conditions. The test therefore provides relatively imprecise results. "

The salt spray test cannot create real corrosion conditions for several reasons:

  • The surface of the samples is constantly moist and does not dry off in between. That alone does not correspond to the real conditions. Metals such as B. Zinc cannot form a passive protective layer in the test as they do under real conditions.
  • The chloride content in the spray is very high (usually 5% NaCl), which means that corrosion is greatly accelerated. However, different metals and metal components have different susceptibility to different acceleration factors.
  • The environmental conditions generated in the test are not realistic and are harder than is the case with normal outdoor use.
  • In the salt spray test, the harmful influence of UV light on coated surfaces is completely ignored, although this is usually the most important cause of the deterioration of coated surfaces.
  • When comparing different metallic coatings, the salt spray test provides results that differ greatly from weathering under real conditions. International standards such as DIN EN ISO 14713-1: 2010-05 "Zinc coatings - guidelines and recommendations for protecting iron and steel structures from corrosion" therefore state that short-term tests such as the salt spray test must not be used for material comparisons and that results from short-term tests are not used Derivation of statements about the corrosion protection duration can be used. In DIN EN ISO 14713-1, for example, it says: "Using salt spray tests on steel with a zinc coating cannot achieve a realistic result because these spray tests wrongly accelerate the failure mechanism."

In some industries, the salt spray test is no longer used today.

application

Some typical corrosion protection systems that can be assessed with this test are:

  • Zinc flake coatings according to ISO 10683 (see zinc flake coating )
  • Organic thin coatings
  • Powder coating

The corrosion protection of coating metals, e.g. B. zinc on steel, can usually not be evaluated meaningfully with the NSS, because the corrosion protection of galvanized surfaces is based on the formation of protective cover layers that are created by natural weather conditions on the surface of galvanized steel parts and consist mainly of basic zinc carbonate. However, these protective cover layers cannot develop in the salt spray test.

If the pure barrier effect of the coating metal is checked up to the onset of base metal corrosion, there is often a direct correlation to the applied weight or the coating thickness. Furthermore, the corrosion products formed quickly and in large quantities are themselves such a major impairment that the incipient base metal corrosion can hardly be regarded as primary damage. In contrast to natural corrosion processes, which can lead to the formation of compact passive layers due to alternating cycles of wetting and drying, there are no corrosion-slowing surface layers in the NSS and comparable, permanently moist tests.

swell

  • ASTM American Society for Testing of Materials. ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus , 2003
  • ISO International Organization for Standardization. ISO 9227 Corrosion tests in artificial atmospheres - Salt spray tests , 2006 (replaced by)
  • ISO International Organization for Standardization. ISO 9227 Corrosion tests in artificial atmospheres - Salt spray tests , 2012

Individual evidence

  1. ↑ DIN EN ISO 14713-1 - Zinc coatings - Guidelines and recommendations for protecting iron and steel structures from corrosion - Part 1: General design principles and corrosion resistance (ISO 14713-1: 2009); German version EN ISO 14713-1: 2009, page 25
  2. ↑ Townsend, H. E. 'Development of an Improved Laboratory Corrosion Test by the Automotive and Steel Industries', Proceedings of the 4th Annual ESD Advanced Coating Conference, Dearborn, MI (USA), November 1994, ESD Ann Arbor, MI
  3. ↑ Appleman, B. 'Cyclic Accelerated Testing: The Prospects for Improved Coating Performance Evaluation', J Protective Coatings & Linings, p71-79. Nov 1989
  4. ↑ Appleman, B. 'Cyclic Accelerated Testing: The Prospects for Improved Coating Performance Evaluation', J Protective Coatings & Linings, p71-79. Nov 1989
  5. ↑ DIN EN ISO 14713-1 - Zinc coatings - Guidelines and recommendations for protecting iron and steel structures from corrosion - Part 1: General design principles and corrosion resistance (ISO 14713-1: 2009); German version EN ISO 14713-1: 2009, page 25