Atmospheric exposure tests specimens

In order to secure information as to changes in corrosion rates with time, as in atmospheric exposure tests, it is necessary to expose sufficient specimens to allow sets to be taken from test after at least three time intervals. 

In view of possible or probable variations in mechanical properties among different specimens of the same metal cut from different sheets or other pieces, or even from different sections of the same sheet or piece, it is necessary to pay careful attention to the initial sampling of stock to be used for control, as well as exposure, specimens. An interesting case in which several of these considerations were involved was provided by the long-time atmospheric exposure tests of non-ferrous metals carried out by Subcommittee VI of ASTM Committee B-3 on Corrosion of Non-Ferrous Metals and Alloys in which changes in tensile properties were used as one of the means of measuring the extent of corrosion. 

Exposure Test specimens were of AISI Type 316L. (UNS S31603) stainless steel (density 7.98 g/cm ), with dimensions of 25 x 50 x 3 cm (1 X 2 X % in.), immersed in 500 mL of 15 % methylsulfonic acid + 5 % sulfuric acid, balance water, at atmospheric pressure and 45°C. 

Another variation of the immersion test is the cyclic test procedure where a test specimen is immersed for a period of time in the test environment, then removed and dried (either air diy or use of heat lamps), then re-immersed to continue the cycle. Normally hundreds of these cycles are completed during the course of the test program. In ASTM G 60, test specimens are dipped in a test solution before being exposed to atmospheres varying in relative humidity. The intent of the test is to develop a layered corrosion product similar to that found on sheltered outdoor exposure test specimens. These tests can be either manually performed or conducted in apparatus equipped for automatic cycling. 

The precautions generally applicable to the preparation, exposure, cleaning and assessment of metal test specimens in tests in other environments will also apply in the case of field tests in the soil, but there will be additional precautions because of the nature of this environment. Whereas in the case of aqueous, particularly sea-water, and atmospheric environments the physical and chemical characteristics will be reasonably constant over distances covering individual test sites, this will not necessarily be the case in soils, which will almost inevitably be of a less homogeneous nature. The principal factors responsible for the corrosive nature of soils are the presence of bacteria, the chemistry (pH and salt content), the redox potential, electrical resistance, stray currents and the formation of concentration cells. Several of these factors are interrelated. 

In all the paint systems tested in the atmosphere, the presence of the chloride contaminant at its highest concentration, causes the appearance of small blisters. Such blisters may in time burst due to the oxide built-up inside them, as was seen in the case in the CR system ("Table III.") In the P system ("Table III."), the blistering seen with the third level of chlorides continues after two years outdoor exposure, without rust coming through the coating. This same effect is also seen in salt fog test panels, but it does not appear in those test specimens subjected to accelerated weathering tests. 

The test specimens were suspended in five different test tanks. One tank had 1.0 N NaOH, two had 0.25N NaOH and two had 3.5% (by wt.) NaCl solution. One 0.25N NaOH and one 3.5% NaCl solution tank were purged of oxygen with a nitrogen atmosphere (oxygen concentration of less than 3%), the complimentary pair of tanks had natural air exposure (oxygen concentration 20%). A summary of the initial status of each tank is given in Table I. 

Open-air exposure involves direct exposure to all atmospheric conditions and contaminants. Sheltered exposure is exposure with protection from atmospheric precipitation and solar radiation either under a cover or in a partly closed space such as shutter sheds. The total period and the season of exposure depend on the type of test specimen and the purpose of the test. The evaluation is carried out by visual examination, metallographic examination and estimation of mass loss, change in mechanical properties or performance characteristics of the test specimens [84]. 

Other environmental related tests include ASTM D-904, Standard Practice for Exposure of Adhesive Specimens to Artificial (Carbon-Arc Type) and Natural Light ASTM D-1828, Standard Practice for Atmospheric Exposure of Adhesive-Bonded Joints and Structures ASTM D-1879, Standard Practice for Exposure of Adhesive Specimens to High Energy Radiation and ASTM D-3310, Standard Test Methods for Determining Corrosivity of Adhesive Materials. 

Service (plant) and field testing, including exposure of coupons, test specimens or components in process environments of industrial plants as well as in special natural fields (in seawater, atmospheres, soils etc.)... 

The adhesion of specimens subjected to environmental exposure tests was evaluated prior to and subsequent to the contact. Wet thermal shock testing consisted of five cycles each for IS minutes in boiling water and 2 minutes in ice water. The maximum transfer time between the two baths was 30 seconds. Thermal and temperature/humidity exposures were performed in controlled atmosphere chambers for 200 hours. Simulated solder tests comprised immersing the test. specimen in a silicone oil followed by solder flotation. The solder temperature varied from 232 to 288 C and the contact time was S or 10 seconds. In some cases, two flotations were performed on the same sample. The effect of a heat treatment at 135 C prior to solder testing was examined the heat treatment time varied from 0 to 16 hours. ... 

Section HI covers Types of Tests (H. Hack, Section Editor) includirrg laboratory-accelerated tests, field tests, and service tests. The chapters in this section provide basic principles, describe test techniques and specific considerations such as specimen preparation, test duration and acceleration factors, and cite pertinent standards. Chapters included under laboratory tests are electrochemical, cabinet, immersion, high temperature, and high pressure. Field Tests chapters include atmospheric exposure, seawater, fresh water, and soil. Under service tests are industrial applications and high temperature environments. 

The type of laboratory immersion test to be used will be determined mostly by the environmental conditions that are to be simulated. For example, if the equipment is immersed in service, then the test specimens should be immersed in the laboratory test if the exposure is alternating immersion and atmospheric exposure, then a cyclic exposure test to wet/dry conditions should be used. Another determining factor is the duration of exposure for arriving at the desired results, which is associated with the degree of acceleration that will be required of the test method. 

Probably the most common type of atmospheric exposure is conducted in a static location that is, racks or holding devices are fixed in a location and specimens are exposed for a test period at that site. This type of exposure can be ctirried out in sheltered sites or boldly exposed sites. A wide variety of different types of specimens may be employed in long-term tests. 

The type of preparation that a specimen will require depends very much on purposes of the exposure program [3]. In cases where bare metals are to be exposed to the atmosphere, a somewhat arbitrary decision must be made on what type of surface finish will be tested. Typical steps in an atmospheric exposure program are given in Table 9. 

After removal at the specified exposure period, the specimens are evaluated. Visual examination of the specimens upon removal is usually performed, and a photographic record of appearance is valuable. ASTM G 33, Practice for Recording Data from Atmospheric Corrosion Tests of MetaUic-Coated Steel Specimens, can provide guidance for evaluation procedures. Before determination of mass loss, or for most other evaluation techniques, the specimens must be cleaned. Again, cleaning procedures are described in ASTM G 1 and A 380 for most materials of interest. ASTM G 1 also includes a procedure for determining when the corrosion products have been removed. After cleaning, specimens should be placed in a desiccator with fresh dessicant until ready for further evaluation. 

G33 Atmospheric Corrosion Tests of Metallic-Coated Steel Specimens practice for exposure programs. 

The atmospheric exposure outlined in ASTM G 50 may also be useful for SGG testing. Recommended exposure conditions, including solution composition, temperature, and drying cycles are listed. The time to first appearance of cracking is recorded. The stress state to be tested depends in large part on the geometry of the actual part, and methods to apply pcuticular stress states to test specimens will be described in later paragraphs. 

Atmospheric exposure consists of placing specimens such as coupons, parts, components, on racks at stationary test sites [53]. The example in Fig. 7 shows bumpers imder test at the marine splash and spray facility of the LaQue Center for Corrosion Technology at Wrightsville Beach, North Carolina. The nature of the site varies depending on geographic area and specific location within an area. Factors such as environmental chemistry (chlorides, pollutants. 

Standard Practice for Testing Water Resistance of Coatings Using Controlled Condensation Standard Practice for Atmospheric Environmental Exposure Testing of Nonmetallic Materials Standard Practice for Recording Data from Atmospheric Corrosion Tests of Metallic-Coated Steel Specimens... 

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