ABSTRACT

The ASTM G 48 test evaluates an alloys resistance to pitting and crevice corrosion using severe test conditions. Unlike most other ASTM standards, which are cut and dry in their test procedures, this standard has loopholes, which allows test-to-test and laboratory-to-laboratory variation, which makes comparison of data difficult. Modifications are often used to improve the standard, but these are not adopted universally. 

This paper identifies the problems associated with using G 48, and the impact these problems have on evaluating high-alloy stainless steels. Recommendations to correct these problems to standardize the test are made. Attention to the use of electrochemical measurement techniques for improved reproducibility are discussed. 

KEYWORDS: ASTM G 48, critical crevice temperature, critical pitting temperature, crevice corrosion, electrochemical testing, ferric chloride, pitting, stainless steel, temperature, testing

 

INTRODUCTION 

The shortage of capital project money for building new process facilities that require highly corrosion resistant materials is forcing design and project engineers to more precisely assess performance versus pricing, now more than anytime since World War II. The heavy global competition among the companies producing high performance alloys and stainless steels require those companies to make their products yet more attractive to the end users. 

The recognized need for materials suitable for the fabrication of equipment employed in corrosive environments has stimulated research and development of new stainless steels and nickel-based alloys, as well as modification of the existing ones. However, it is becoming more evident that usually if failure of a high performance alloy occurs, it is due to localized attack, i.e., pitting and crevice corrosion. Although significant attention has been given to the design of alloy composition and marketing with respect to specific industries, the overall rating of high performance alloys on the basis of an objective standard criterion for localized attack still remains open. 

The principal alloying elements for corrosion resistance of the high-alloyed stainless steels are increased chromium content, which provides progressive resistance to general attack; and increased molybdenum (together with small additions of nitrogen), which enhances resistance to localized attack. Considering the higher prices commanded by the materials with high alloy content, the selection of material for a given service would not necessarily be based only on corrosion resistance, but on cost effectiveness! Thus, the most common high performance alloys and stainless steels on the market, having 16-28% Cr and 1-16% Mo, vary in price from approximately $2.00/lb (Type 316L SS) to approximately $10.00/lb (Alloy C-276). 

Some of these alloys, despite high nickel, chromium, and molybdenum contents, may be susceptible to localized corrosion in certain environments. Comparison studies performed in the past ten years to rank the wide variety of commercially available high performance alloys are usually either oriented to a specific newly developed alloy, or are "industry specific" and based on specific "in-situ" field data rather than standardized laboratory results. 

Often the need for immediate results precludes the luxury of testing candidate alloys in plant tests (which may require months) or the use of laboratory tests especially designed to simulate plant conditions. For example, standardized tests, such as ASTM G 48 ("Standard Test Methods For Pitting And Crevice Corrosion Resistance Of Stainless Steels And Related Alloys By The Use Of Ferric Chloride Solution"), often provides results on performance under very severe testing conditions in a short length of time. This particular test is based on the knowledge of the localized corrosion mechanism. 

There are two test methods currently covered in the present G 48 standard: Method "A" calls for the total immersion of a test specimen in a 6 wt.% ferric chloride solution (10% FeCl₃ • 6H₂O) and evaluates the susceptibility to pitting corrosion; Method "B" utilizes a creviced test specimen immersed in a 6 wt.% ferric chloride solution and evaluates crevice corrosion susceptibility. Unfortunately, using this standard test does not always achieve standard testing conditions. Considerable variability has been observed for the newer high-alloy stainless steels from test-to-test and between laboratories. These situations are attributable to variance in: solution pH, specific test temperature, specific testing times, criteria for failure, compressive pressure on the crevice washer, etc. Additionally, many modifications to these test methods are frequently employed, such as, deionized water replacing distilled water; weight loss criteria instead of pit presence/density/depth; alternate crevice devices/composition. It is problems such as these, which this paper will address, plus alternative uses/refinements of the standard.

OVERVIEW OF GENERIC PROBLEMS

The Ferric Chloride Solution 

Water. The G 48-76 standard states that distilled water be used to make up the test solution, however it does not specify to what purity or pH. ASTM D 1193³, "Standard Specification for Reagent Water," specifies four Types of water based on purity and pH. For the purposes of testing to G 48, either Type III or IV may be suitable, however these waters include those produced by either distillation, ion exchange, or reverse osmosis. Likewise, the pH may range from 6.2 to 7.5 for Type III, or from 5.0 to 8.0 for Type IV. 

Since the current standard G 48 only specifies "distilled" water, the purity can be somewhat assured, but the pH could be as low as 5 or as high as 8.0, depending upon how much carbon dioxide is dissolved in the water. Although carbon dioxide is driven out of the water during boiling, it dissolves back into solution upon condensing lowering the pH of the water. 

pH. The pH of the as-made ferric chloride solution has been reported^4-10 to range from 1.0 to 2.05 and even greater than 3.⁴ This much of a variance may greatly affect the alloys pitting or crevice susceptibility. In addition, during the course of testing, the pH of the bulk solution has been known to change, particularly when temperatures change. For example, increasing the temperature of the ferric chloride solution decreases its pH. At a critical temperature of 45° C ferric hydroxide begins to precipitate and the solution rapidly becomes more acidic, see Figure 1. This change in solution pH will affect its corrosivity, particularly to lesser alloyed stainless steels.

In the ASTM G01.05.07 Task Group on Critical Pitting and Crevice Temperature Round Robin testing¹¹ , the addition of hydrochloric acid to a 1% concentration was used to control and normalize the 6 wt.% ferric chloride solution pH to approximately 0.4 throughout the testing time. 

Others believe this solution is too severe to evaluate sigma phase in UNS S31803, and wish to adjust the pH to a constant 1.3¹² by either the addition of HCl or NaOH. 

Duration Of Test 

The G 48 standard suggests a "reasonable test period of 72 hours," although this is not a requirement, and a number of researchers ^{2, 4-7 , 9 ,13-20} have reported using other test periods in the generation of pitting and crevice corrosion susceptibility data.

Temperature For Evaluation 

The results of G 48 immersion tests (carried out at fixed temperatures of 22° or 50°C)_, with rare exceptions, are judged to be a "go/no-go" criterion, and are not alloy specific. 

In order to be able to rank a material of interest, or select a temperature criterion for material acceptance, it is common to determine the lowest temperature at which pitting/crevice initiates. This criterion is called the Critical Pitting or Critical Crevice Temperature (CPT or CCT, respectively^{13, 16}). The development of standard immersion testing methods using this concept are the subject and charge of the ASTM G01.05.07 Task Group.

Criteria For Failure 

ASTM G 48 does not address a criterion for either acceptance or rejection, only the examination of the tested specimen for the presence or absence of pitting/crevice attack. Currently, the criterion is left to the vendor and user to agree upon. 

SPECIFIC PROBLEMS ASSOCIATED WITH METHOD A -- THE PITTING TEST

Method A is designed to determine the relative pitting resistance of stainless steels and nickel-base, chromium-bearing alloys. It may also be used to determine the effects of alloying additives, heat treatment, and surface finishes on pitting resistance. 

While no specific problems are associated with this test method, it does lack a specific evaluation criteria. The examination and evaluation section of the standard recommends an ocular review of the exposed specimen under low-power magnification with the subsequent recording of pit depth and pit density. Weight loss calculations are to be reported as grams per square meter. Comparisons between alloys are to be ranked according to these results, however this method is generally applied as a "go/no-go" test. Because of the possible subjectiveness in identifying a pit, it has been recommended¹⁰ that a corrosion rate criterion be adopted. This would not only create a specific numerical acceptance criterion, but would also eliminate problems associated with mis-identification of a mechanically damaged site as corrosion pitting. 

Almost 20 years ago, Brigham and Tozer¹³ proposed using temperature as a means to rank certain molybdenum containing alloys, and to determine their critical pitting temperature below which the alloy would not pit regardless of exposure time or potential. Although several chloride containing test media were used giving statistically identical results, the ferric chloride solution was preferred owing to its simplicity. Their continued work, plus the works of others ^{4, 21}, contributed to the development of the G 48 standard. Unfortunately, the criterion of the critical pitting temperature was not developed. However, this is exactly the charge of the ASTM G01.05.07 Task Group, who will hopefully have a proposed revision/addition to the G 48 standard in 1993. 

SPECIFIC PROBLEMS ASSOCIATED WITH METHOD B THE CREVICE CORROSION TEST

In 1974, both Streicher⁴ and Brigham¹⁶ published works on establishing a crevice test using the ferric chloride solution. The work of Streicher became the basis of Method B, where two tetrafluoroethylene (TFE-fluorocarbon) blocks were assembled, one on each side of the test specimen, and held in place by O-rings or rubber bands. Unfortunately, this assembly, as described in G 48, has several drawbacks, namely: the breakage of an O-ring or rubber band which invalidates the test; non-uniform compressive pressure associated with the O-ring or rubber band; the creviced area-to-bold area ratio²² cannot be accurately measured; the TFE-block surface finish is not specified; and the crevice attack almost always initiates on the edges of the specimen in contact with rubber bands rather than under the TFE blocks. 

In 1976, Anderson²² published results obtained using a multiple crevice washer that was bolted to the test specimen. This assembly is generally accepted as an improvement to the TFE blocks because it eliminates the use of rubber bands, which can break during testing, a precise creviced area-to-bold area ratio can be calculated, and the edges of the test specimen are not disturbed. This assembly has- been adopted by MTI ²⁰ as the method for evaluating the relative resistance of alloys to crevice corrosion. But even this method does not go without problems. For example, there has not been an established standard torque to which to tighten the crevice assembly, the use of various materials of construction (e.g., TFE, acetal resin, polycarbonate, methyl methacrylate, ceramic alumina) for the crevice washer is not defined, and the relaxation of torque during the test^23-25 is not addressed.

 Compressive Pressure Of The Crevice Device

As mentioned above, the G 48 standard utilizes cylindrical TFE-fluorocarbon blocks and fluorinated elastomer O-rings or rubber bands. The degree of pressure exerted by the O-rings/rubber bands is not precise or reproducible, and hence non-standard. In an attempt to standardize the degree of crevice tightness for the multi-crevice washer assembly, the use of a torque wrench was used to tighten the bolts. But, here again, the standardization vanishes. Torques ranging from 0.28 N-m (2.5 in-lb) to 8.5 N-m (75 in-lb) have been reported ^{2,5,6,9,20,24-28} in use with the ferric chloride solution.  

The limitations of using torque as a mean of controlling crevice tightness have been discussed by Oldfield.²³ He has concluded that "the torque cannot be related to a force between the sample and the crevice former." And secondly, the torque can relax with time resulting in a variable force over the duration of the test. The new crevice assembly described by Oldfield²³ shows promise, and hopefully additional test results on high-alloy stainless steels will be forthcoming. 

Another concern in using artificial crevice formers is their geometrical configuration (i.e., its height versus the diameter at the metallic washer, the dimensions of the crevicer, the width of the serration, depth of the serration, etc.). Again, as with the use of torque tightness, there is no standard and crevice geometries vary from laboratory-to-laboratory and even time-to-time by the same experimenter.¹⁸ 

REPRODUCIBILITY 

There are many factors, which are known⁵ to cause localized corrosion of stainless steels, among these are low pH, high chloride concentrations, solutions with noble redox potentials, oxygen differential concentrations, and elevated temperature. Previous workers^{4, 7, 9, 13-15, 2 7} have published that the use of the ferric chloride test will result in the critical pitting temperature varying within the range of 2.5° C. However, using the ferric chloride test, and a variety of crevice-type devices, the best reproducibility for the critical crevice temperature test is within the range of 10 ° C.^{5, 6 ,9,15,16} Therefore, this immersion test has draw-backs when it comes to ranking the crevice corrosion resistance of the high-alloy stainless steels. These drawbacks limit the reliability of using G 48 as an accelerated laboratory test. 

Electrochemical measurement techniques for evaluation of crevice corrosion are not new^{9, 13}, however, they are tedious to run manually. However a personal computer system has been developed which automatically apply potentials and measure the resulting corrosion current, while ramping and controlling solution temperature. Critical pitting and critical crevice temperatures can now be determined in less than one hour. Plus, the use of such instruments has resulted in both the critical pitting and crevice temperature determination varying only 2.5 ° C. These preliminary results^{5, 6, 29, 30} are significant enough to rank not only high-alloy stainless steels of different families (i.e., 2% Mo, 4% Mo, 6% Mo, 8% + Mo), but alloys within the same family.

RECOMMENDATIONS 

The specific use of "distilled water" in G 48 is limiting and without justification. As observed in our laboratory, Table 1, the purity of deionized water can often exceed that of distilled water. Therefore, as an improvement to G 48, it is recommended that the standard solution be composed of 100 g of reagent grade ferric chloride hexahydrate (FeCI₃ • 6H₂O), in 900 ml of reagent water (ASTM D 1193, Type IV). [NOTE: Current balloting for the revision/reapproval of ASTM G 48-76 will specify the use of ASTM D1193 instead of "distilled water."³¹] 

Furthermore, since the pH of the as-made ferric chloride solution appears to vary from laboratory-to-laboratory, it is also recommended to adjust the pH of the above solution with 15.6 mls of reagent grade concentrated hydrochloric acid. This will produce a solution containing 6% ferric chloride by weight and 1% hydrochloric acid, resulting in a stable and buffered pH (below pH 1) environment for temperatures ranging from ambient to 85^oC, as determined from our laboratory test results, Table 2. 

The G 48 standard only suggests a test time of 72 hours, while a number of others ^{2, 4-7, 9, 13-20, 32} have successfully used anywhere from 23 hours to 30 days to achieve desired results. Based on the analysis of these results, and our work presented in Table 3, it is recommended that a standard test time of 48 hours be adopted. 

Because G 48 is specifically related to localized attack, one could assume that most, if not all, corrosion is associated with pitting/crevice attack. The unpublished results of an extensive testing program for a nuclear facility, suggest that during a 48 hour test the degree of pitting attack is directly related to molybdenum content for properly annealed material. When the molybdenum content is above 4% the pits are shallow and occluded. This made pit depth measurements difficult and subjective. Thus to evaluate the high-alloy stainless steels in the pitting test, Method A, a mass loss per unit area per time of exposure criteria would normal­ize a small number of deep pits or a massive amount of shallow pits. As such, certain indus­tries have stipulated a criteria of less than 10 mdd (milligrams loss per square decimeter per day) as an acceptable degree of attack for the acceptance or rejection of material for chloride service. In order to rank alloys, a comparison of this rate should be made. 

For crevice attack evaluation, MTI²⁰ has established the criterion of the number of crevice sites, which are attacked to a depth greater than 1.0 mils during the 24 hour test. Certain other industries²⁸ accept material only if all of the crevice sites are attacked to a depth of less than 1.5 mils in a 72 hour test. Since the attack of one (1.0) mil is readily observed by ocular examination, and the measurement of one mil is easily achieved by either a needle-point micrometer or a calibrated fine-focus microscope³³, it is recommended that this criteria be accepted for the test period used. 

Crevice assemblies vary from TFE blocks held in place with either O-rings or rubber bands, to serrated washers of a non-metallic material bolted through a hole in the test specimen, to non-metallic washers with a controlled annulus, spring loaded to the test specimen. There are advantages and disadvantages to each design, however, the most popular, widely used, and recommended by the author is the multiple crevice assembly consisting of two TFE serrated washers (16 mm in dia., 6.7 mm dia. center hole, with 12 crevice pads of 1.45 mm in width by 0.8 mm in height), which are attached to the test specimen with a nut, bolt and washers of Alloy C-276 (UNS N10276). An insulating sleeve should be used around the bolt to electrically isolate it from the test specimen. The pads of the washer in contact with the test specimen should be polished to a 600-grit finish prior to assembly. The torque on the bolt should be 0.28 Nm (2.5 in-lb), unless crevice tightness characteristics are being studied. Furthermore, unless a specific surface condition of the test specimen is to be evaluated, the specimen should be ground to a 120-grit finish. For each temperature of evaluation, a new specimen and fresh solution should be used.

CONCLUSIONS 

The ASTM G 48 standard needs to be improved, and loopholes in the testing conditions and procedures eliminated. Specific problems with the make-up of the ferric chloride test solution, the duration of the test, evaluation temperatures, and a criterion for failure have been addressed and recommendations to improve the standardization the test are made.

The ASTM G01.05.07 Task Group is working on revisions/additions, which will provide versatility in using this standard under clearly defined test parameters.  

The use of personal computers, programmed to control many test parameters concurrently, in combination with the ferric chloride test solution, offers a rapid determination of either the CPT or CCT, with excellent reproducibility.

REFERENCES