Sulfur Dioxide Tesing – Corrosion of Metals and Alloys – ISO 22479

Sulfur dioxide (SO2) is a highly reactive gas that is commonly released during the combustion of fossil fuels and industrial processes. It is known for its corrosive properties, especially on metals and alloys. Understanding the effects of sulfur dioxide on corrosion is crucial, particularly for industries where exposure to SO2 is prevalent. In this article, we will delve into the topic of sulfur dioxide testing and its impact on the corrosion of metals and alloys, specifically focusing on ISO 22479 standards.

Sulfur dioxide testing involves the assessment of materials’ resistance against corrosion caused by exposure to SO2. This testing provides valuable insights into the durability and performance of metals and alloys in environments where sulfur dioxide concentrations are high. The primary aims of sulfur dioxide testing are to understand the extent of corrosion, identify potential weak points in materials, and develop preventive measures to mitigate SO2-related corrosion.

Kesternich test chamber
SO2 corrosion test chamber
The preferred capacity of the test cabinet is (300 ± 10) 1.When agreed between the interested parties, other capacities may be used. In these cases, the size and/or number of test specimens, the volume of gas and the quantity of water shall be properly arranged in accordance with the capacity of the test cabinet. 5.2 Test Cabinet
The upper part of the cabinet shall be designed so that drops of condensed water formed on its surface do not fall on the test specimens being tested.An inclination of the upper part of the test cabinet of about 12° or more to the horizontal plane provides a suitable safeguard.The test specimens may be placed at different levels within the cabinet, as long as the solution does not drip from the test specimens or their supports at one level onto other test specimens placed below.
The temperature in the test cabinet is controlled by heating the floor and lower part of the side walls of the test cabinet. The temperature shall be measured at least 250 mm from the side walls and at least 150 mm below the upper part (lid).
A gas inlet port shall be less than 50 mm above the water surface.The test cabinet shall be hermeticallyleak-tight.
A pressure relief valve shall be placed in or near the upper part of the test cabinet.
The exhaust gas and the released gas from the pressure relief valve shall be treated as appropriate.
Relevant regulatory limitations can apply. Note:
A drain port shall be provided in the test cabinet.
A typical test cabinet is show in Figure 1. Another example of test cabinet is shown in Figure A.1.
The test cabinet shall be installed in a room free from dust, draughts, corrosive gas and direct solar radiation, at a room temperature of (+23℃ ±5℃) and at a relative humidity of less than 75%.
Figure 1 - Typical Test Cabinet
The source of sulfur dioxide should be a gas cylinder with a volume concentration of more than 99,9 %. It shall be fitted with appropriate regulating and measuring apparatus to ensure the supply of the correct volume of gas. The volume of gas delivered into the test cabinet should be measured by a calibrated flow meter. Another measuring instrument works with squeezing out a viscous liquid paraffin for the volume of the gas for 0, 2, 1,0 and 2,0 1iters. 5.3 Source of Sulfur Dioxide
Warning – Sulfur dioxide (CAS no.7446-09-5) is toxic, corrosive and irritating.Handling of sulfur dioxide shall be restricted to skilled personnel or conducted under their control.The apparatus shall be used and maintained by skilledpersonnel, not only so that the procedures can be performed correctly, but also because of the hazards to health and safety that are involved.
In order to minimize the effect from the material of the cabinet by sulfur dioxide, a new cabinet shall be operated at least one test cycle in accordance with the procedures given in 8.2 to 8.5 without test specimens. 5.4 Conditioning of a New Cabinet.
The conditioning shall conform to Table 1, with 2,0 liters of sulfur dioxide.
The number, type, surface roughness, thickness of coatings, shape and dimensions of test specimens shall be selected according to the specification for the materials or product beging tested. When not specified, details concerning the test specimens shall be agreed between the interested parties. 6.1 General 6. Test Specimens
A typical specimen size is 150mm*100mm by 0.75mm to 1.25mm thickness. 6.2 Dimensions
Thoroughly clean the test specimens before testing. The cleaning method depends on the nature of the surface and the contaminants. Abrasives shall not include the use of any abrasives or solvents that may attack the surface of the test specimens. Take care that the test specimens are not recontaminated after cleaning, by excessive or careless handling. 6.3.1 Metals and Alloys 6.3 Preparation
lf test specimens are cut from a large coated article, the cutting shall be carried out in such a way that the coating is not damaged, especially in the area adjacent to the cut. Unless otherwise specified, the cut edges should be adequately protected by coating with a suitable medium that is stable under the conditions of test, such as wax or adhesive tape.
Unless otherwise specified or agreed, the test specimens shall be prepared in accordance with IS0 1514 and then coated by the specified method with the product or system under test. The back and edges of the test specimens should be coated with the product or system under test. 6.3.2 Paints and Varnishes
Dry (or cure) and age (if applicable) each coated test specimen for the specified time and under the specified conditions and, unless otherwise specified, condition them at a temperature of (23± 2) °C and a relative humidity of (50±5) % for at least 16 h, with free circulation of air and not exposed to direct sunlight.The test procedure shall then be carried out as soon as possible.
Determine the thickness, in micrometres, of the dried coating by one of the non-destructive proceduresdescribed in ISO 2808.
The distance between test specimens shall be not less than 20 mm. The distance between the edge of the test specimens and a wall of the test cabinet shall be not less than 100 mm.The test specimens shall be arranged in such a way that they cannot protect each other from the influence of the environment. The distance between the lowest edge of the testspecimens and the surface of the water shall be greater than 200 mm.The contact area between test specimens and their supports shall be as small as possible. 6.4 Arrangement of the Test Specimens
The orientation of the exposed test surface in the test cabinet is critical.Unless otherwise specified,test specimen shall be mounted at an angle between vertical and (15 ± 2)° to the vertical, with the area of primary interest facing up.
Note: This is common practice for testing metallic specimens and measures to protect them from corrosion. When testing assemblies or enclosures, it is preferable to mount the specimen at the same angle as in typical use.
The total exposed surface area of the test specimens for one test shall be (0,5 ± 0,1) m2 for each (300 ± 10)1 of the test cabinet. For different volumes of the test cabinet, the total exposed surface area is to be adjusted proportionally. When the total exposed surface area is less than 0,5 m2 for 300l of the cabinet, substitute specimens shall be added to obtain a total surface area of 0,5 m2.The materials of the substitute specimens shall be the same as of the test specimens. Otherwise, test results may be different, since the influence (absorbance) of sulfur dioxide differs by materials.
One test cycle is 24 hours, either Method A or Method B. 7.1 Test Cycles 7 Test Conditions
In Method A, test specimens are exposed to sulfur dioxide for 24 hours. If test specimens are affected seriously by the temperature and humidity, Method B should be chosen.
In Method B, test specimens are exposed to sulfur dioxide for 8 hours, followed by exposure to a standard atmosphere for 16 hours.
The test conditions of Method A and Method B are shown in Table 1 and Table 2, respectively.
For metallic and non-organic coatings, 0.2 liters volume of sulfur dioxide should be used.For paint and varnish,0,2lor 1,0lvolume of sulfur dioxide should be used.Generally,0.2 liters volume of sulfur dioxide is recommended where the coating thickness does not exceed approximately 40 um 2 liters volume of sulfur dioxide is used by agreement between the interested parties or according to product specifications, such as corrosion resistant coatings (e.g.hard chrome plating, anodised coating).
Table 1 - Test conditions of Method A
Table 2 - Test conditions of Method B
Note 1: The oretical concentrations of sulfur dioxide at the beginning of each test cycle are 0.067 %, 0.33 % and 0.67 %, corresponding to 0.21, 1,0l and 2,0l of sulfur dioxide gas, respectively. However, much of the sulfur dioxide is quickly dissolved into the water at the bottom of the test cabinet. Thus, the effectivereal sulfur dioxide concentration in the gas cabinet is much lower than the theoretical concentration (seeFigure C.1).
Note 2: The ± tolerances given for the temperature are allowable fluctuations, which are defined as the positive and negative deviation from the setting of the sensor at the operational control set point during equilibrium conditions. This does not mean that the set value can vary by plus/minus the amount indicated from the given value.
The test duration shall be as designated by the specification for the material or product being tested. When not specified, the test duration shall be agreed between the interested parties. 7.2 Test Duration
The test may be finished if a specified degree of corrosion has been reached, or if the appearance or function of test specimens has otherwise been impaired to an unacceptable level.
Recommended numbers of test cycles are: 1, 2, 5, 10, 15 or 20.

ISO 22479 is an international standard that provides guidelines for evaluating the corrosion resistance of metals and alloys in highly corrosive atmospheres, including those contaminated with sulfur dioxide. This standard is widely recognized and adopted by various industries to ensure the reliability and quality of materials used in their processes. It outlines specific test methods and requirements for conducting sulfur dioxide corrosion tests, allowing for standardized evaluation and comparison of different materials’ performance.

Sulfur dioxide is a potent oxidizing agent that reacts with moisture in the air to form sulfuric acid. The presence of sulfuric acid greatly accelerates the corrosion process, leading to the degradation of metals and alloys. Effects of Sulfur Dioxide on Corrosion:
Several factors determine the severity of sulfur dioxide-induced corrosion, including temperature, humidity, concentration, exposure time, and the specific material’s composition.
While SO2 can cause corrosion in a wide range of metals and alloys, some are more susceptible to its effects than others. Stainless steels and nickel-based alloys, known for their excellent corrosion resistance in various environments, can withstand sulfur dioxide corrosion to a certain extent. Corrosion of Metals and Alloys:
However, Oother metals like copper, brass, and carbon steel are highly vulnerable to SO2-induced corrosion and can suffer substantial damage even from low concentrations of the gas.
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