Accelerated tests problems

Ketcham, S. J. and Jankowsky, E. J., "Developing an Accelerated Test Problems and Pitfalls, Laboratory Corrosion Tests and Standards, ASTM STP 866, G. S. Haynes and R. Baboian, Eds., ASTM International, West Conshohocken, PA, 1985, pp. 14-23. 

S. J. Ketchum and E.J. Jankowsky. 1985. Developing an accelerated test Problems and pitfalls, ASTM STP 866 Laboratory Corrosion Tests and Standards, Philadelphia American Society for Testing and Materials, pp. 14—23. 

Corrosion Resistance. The environment to which a plated part is to be exposed should be a part of any definition of corrosion resistance. Problems arise in testing a part in its intended environment in part because of the long time period required. In many plating processes, corrosion resistance is direcdy proportional to the plate thickness, so a specification on plate thickness is a much faster method of indirectiy measuring corrosion resistance. In specifying a corrosion resistance requirement for the production of plated goods, accelerated tests are used especially if plate thicknesses caimot be related to corrosion protection. 

There are a number of intrinsic problems with accelerated tests used to assess lifetime. Firstly, the general problems in assessing service life mentioned in Section 1.2 apply. Regardless of how you go about making assessments, service conditions are not constant and, with long expected lifetimes, either the degree of extrapolation is very large or even accelerated tests need considerable time scales. 

The problems particular to accelerated tests are related to the extrapolation process. It was stated earlier that it is essential that extrapolation rules from the test conditions to those of service are known and have been verified. In practice this is only an ideal as extrapolation procedures have not generally been comprehensively validated and almost certainly will not give accurate predictions in all cases. The only choice is to use the best techniques available and apply them with caution. 

The effect of ionising radiation is described in Section 4.2. Most often, accelerated tests are carried out using gamma radiation from an isotope source or an electron beam from an accelerator. Radiation from nuclear reactors can also be used but will be a mixed radiation which may or may not be suitable for the simulation. The penetration of an electron beam is inherently limited which means that only relatively thin samples can be treated. Hence, gamma irradiation is the more versatile technique. With thin samples, such that penetration limits are not a problem, there are conversion factors to approximately equate the various radiations and energies to an equivalent gamma dose. 

Anticipated problems and counter measures for the listed protocols have been included in individual steps of the basic protocols. In addition, a detailed troubleshooting section for particle size analysis can be found in unit d.u however, the difficulty of properly conducting accelerated tests should again be stressed. While the method is extremely useful, as it can drastically reduce the required time for the overall test, results may not accurately reflect... 

To avoid problems in the field, run a simple accelerated test in which samples are placed in a laboratory oven at an elevated temperature for various times. For example, set the over for 120°F and check for blooming after 24, 48, and 72 h. A visual check against parts stored at room temperature reveals any problems. This simulates the worst storing and shipping temperature conditions and provides a definitive answer in about three days. 

A major problem in automotive vehicles is corrosion control, particularly in the northern areas of the country where salts are used to melt road ice. Protection of exposed steel from corrosion is accomplished by electroplating with copper-nickel-chromium films, particularly on trim and finish moldings. Electrophoretic painting of body panels is used industry-wide, and electrogalvanizing to protect steel under paint is soon to become a standard practice for all vehicles. Electrochemical accelerated tests for the integrity of coatings and for the determination of the concentration of coolants are used extensively. 

There is no good way to predict the rates of the chemical and/or physical changes which occur from accelerated tests. In part, the problem has been that there is no adequate laboratory method to measure these effects over such extended times. Furthermore, accelerated tests (in which materials are e osed to higher intensity UV sources in controlled atmo heres) are of limited value in predicting rates since there is often no reciproci between intensi and time of e osure. 

Before heginning a series of experiments to measure corrosion rates or to study some other aspect of corrosion, it is important to determine clearly what the goal of the work is. Usually, such studies require the use of an accelerated test method to provide answers to problems quickly. Exposure testing in the service environment is an important tool in corrosion engineering, hut the time frame for exposure tests is often too long for timely decisions on design. 

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