Cyclic exposure tests

Organic coatings are commonly evaluated using salt water immersion, salt fog or spray, modified salt exposure tests (e.g., salt fog with added SO2), and various cyclic exposure tests. Humidity exposure and water immersion, and, for many applications, physical resistance tests (adhesion, impact resistance, etc.) are widely used preliminary tests. Standard methods for most of these tests are given in compilations of standard tests such as the Annual Book of ASTM Standards (16). Test methods have been extensively reviewed (e.g., 17-23). 

Figure 12. Schematic illustration of zone of adhesion loss in cyclic exposure test.

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. 

There are several accelerated tests which differ in the selection of light source and cyclic exposure to varying degrees of humidity. Some accelerated tests include salt spray, heat, cold, and other weather factors. 

Nonelectrochemical Methods. Nonelectrochemical methods of studying corrosion include exposure tests of performance and primary film property measurements. Standard exposure tests include salt water immersion (3-5% aq. NaCl, usually at room temperature, sometimes oxygen saturated) cyclic immersion (e.g., salt water immersion alternated with drying periods) salt fog or spray (5% aq. NaCl fog,... 

Cyclic oxidation tests were conducted al 800°C in static laboratory air for up to 1500 cycles. Each temperature cycle consisted of exposure for one hour at reaction... 

ISO 14993 2001. Corrosion of Metals and Alloys - Accelerated Testing Involving Cyclic Exposure to Salt Mist, Dry and Wet Conditions. Geneva, ISO Copyright Office, 2001. 

ISO TC 156 WG7/N137, Accelerated corrosion testing involving cyclic exposure to salt mist, dry and wet conditions. Draft International Standard, International Organization for Standardization, 1998. 

Chemical and Corrosion Resistance The corrosion resistance of CCCs depends on thickness and coating age. Corrosion resistance has been observed to scale with total chromium content [153]. Some studies have found that corrosion resistance does scale with Cr(VI) content [154], while others have found no such correlation [155]. Corrosion resistance is evaluated by continuous or cyclic accelerated exposure testing and electrochemical methods. On aluminum alloys, heavy CCCs will resist pitting for as long as 400 to 1000 h [156]. CCC-coated surfaces will exhibit total impedances of 1 to 2 Mf2 cm after exposure to aerated 0.5 M NaCl solution for 24 h. Such coatings can be expected to withstand 168 h of salt spray exposure without serious pitting [157]. CCCs usually perform well in mild neutral environments, but do not fare as well under... 

In an effort to bridge the gap between furnace oxidation studies and arcjet oxidation tests, ManLabs performed a series offumace studies on UHTCs under various ranges oftime, gas velocity, temperature and pressure. " " " Furnace test conditions covered temperatures between 500°C and 2500°C, times from five minutes to four hours at air flow rates in two regimes, low velocity between 0.3 and 3 m/s and high velocity, between 3 and 75 m/s. Cyclic exposures were also performed for most of the candidates. 

Cyclic tensile testing was done after exposure durability testing [16]. The as-received and an exposed curve (similar time and stress of exposed fiber push out sanqile) are shown in Figure 10. As can be seen from the cyclic tensile tests performed at room temperature both as received and post exposed that the exposed sample has a smaller hysteresis consistent with scenario 2 above. 

The approach to accelerated testing was to use the Atlas Xenon Arc WeatherOmeter. This equipment is fitted with a 6 kW Xenon Arc which with suitable filters provides a UV light source with a spectral distribution almost identical to that of sunlight. Variable atmosphere control for ozone and sulphur dioxide had been fitted to the equipment to enable the machine to simulate as closely as possible the more severe conditions encountered in industrial areas. The WeatherOmeter conditions are thus a severe test of any plastics material since they combine cyclic exposure to UV light, humidity, temperature and atmospheric contamination. Typical exposure conditions are given in Table 26.10 below. 

A configuration frequently used in cyclic stress testing is the reduced area adhesive bond (RAAB) specimen. This is similar to the blister detection specimen, but is 1/2 inch wide and a 1/4-inch hole is drilled through the center of the overlap region this provides more environmental exposure of the bondline. Potter has reported using RAAB specimens to compare the durabilities of several modified epoxy adhesives.Specimens subjected to cyclic stressing failed in fewer hours than specimens held at a constant stress, even when cycled at a lower stress level. He also found that a slow cyclic rate was more damaging than a fast cyclic rate. 

Typical cyclic corrosion tests include one or more steps of salt solution exposure, humidity, drying, and/or ambient conditions. A list of commonly used cyclic tests is included in Table 1. 

The automotive industry has taken a lead role in the development of cyclic corrosion tests that can produce results that better correlate with field exposure. In 1984, a joint effort. Automotive Corrosion and Prevention Committee, began between the Corrosion Task Force of the American Iron and Steel Institute and the Society of Automotive Engineers. Nearly 20 years later, and after considering over 130 available laboratory, proving ground, and outdoor corrosion tests, SAE Surface Vehicle Standard, J2334, Cosmetic Corrosion Lab Test, was published in June 1998 [20],... 

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 results achieved by using the ECT technique have very good correlation with long-term exposure tests in brine as well as the same ranking as found in cyclic polEirization work [55]. ECT test methods have been found to be very effective in detecting rather subtle differences in the composition and microstructure of stainless steels [95,96,103]. 

Alternative atmospheric cabinet simulation tests are available, including ASTM G 87, Practice for Conducting Moist SO2 Tests and ASTM G 85, Practice for Modified Salt Spray Testing. Service in environments where humidity or moisture varies may be simulated by cyclic humidity or alternate immersion tests. See ASTM G 60, Test Method for Conducting Cyclic Humidity Tests and ASTM G 44, Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5 % Sodium Chloride Solution. 

Protection against corrosion of iron and steel in structures—Zinc and aluminium coatings—Guidelines Corrosion of metals and alloys— Accelerated testing involving cyclic exposure to salt mist, dry" and "wet conditions... 

There are numerous options for the production of fog and humidity testing in cabinets in order to asses the corrosion resistance of a broad spectrum of products. The basic humidity test is most commonly used to evaluate the corrosion behavior of materials or the effects of residual contaminants. Cyclic humidity tests are conducted to simulate exposure to high humidity and heat typical of tropical environments. 

T. J. St. Clair The optimized (copolymerized 65/35 BTDA/PMDA 3,3 DABP) system mentioned in the talk has been exposed to cyclic 95% relative humidity for 250 hours (at present) and the lap shear samples are essentially the same as before exposure. Testing is continuing. 

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