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G 31 — 72 Reapproved Standard Practice for Laboratory Immersion Corrosion Testing of Metals1 This standard is issued under the fixed designation G 31; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon e indicates an editorial change since the last revision or reapproval. These factors include specimen preparation, apparatus, test conditions, methods of cleaning specimens, evaluation of results, and calculation and reporting of corrosion rates.
This practice also emphasizes the importance of recording all pertinent data astj provides a checklist for reporting test data. Warning—In many cases the corrosion product on the reactive metals titanium and zirconium is a hard and tightly bonded oxide that defies removal by chemical wstm ordinary mechanical means. In many such cases, corrosion rates are established by mass gain rather g3-72 mass loss.
The values given in parentheses are for information only. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Significance and Use 3. This practice, rather than a standardized procedure, is presented as a guide so that some of the pitfalls of such testing may be avoided.
It is impractical to propose an inflexible standard laboratory corrosion testing procedure for general use, except for material qualification tests where standardization is obviously required.
For proper interpretation astn the results obtained, the specific influence of certain ast must be considered.
If the influence of heat transfer effects is specifically of interest, specialized procedures in which the corrosion specimen serves as a heat transfer agent must be employed 1. If the specific astj of high velocity are to be studied, special techniques must be employed to transfer the 2.
Current edition approved May 1, Originally approved in Last previous edition approved in as G 31 — 72 Close attention and a more sophisticated evaluation than a simple mass loss measurement are required to detect this phenomenon.
G31-722 cannot be evaluated by mass adtm alone. The reporting of nonuniform corrosion is discussed below. It should be appreciated that pitting is a statistical phenomenon and that the incidence of pitting may be directly related to the area of metal exposed. For example, a small coupon is not as prone to exhibit pitting as a large one and it is possible to miss the phenomenon altogether in the corrosion testing of certain alloys, such as the AISI Type series stainless steels in chloride contaminated environments.
This cracking occurs under conditions of applied or residual tensile astn, and it may or may not be visible to the unaided eye or upon casual inspection. A metallographic examination may confirm the presence of stress-corrosion cracking. It is imperative to note that this usually occurs with no significant loss in mass of the test coupon, although certain refractory metals are an exception to these observations.
Generally, if cracking is observed on h31-72 coupon, it can be taken as positive indication of susceptibility, whereas failure to effect this phenomenon simply means that it did not occur under the duration and specific conditions of the test. Separate and special techniques are employed for the specific evaluation of the susceptibility of metals and alloys to stress corrosion cracking see Ref. Alternatively, the coupon may g31-7 rotated through the environment, although it is then difficult to evaluate the velocity quantitatively because of the stirring effects incurred.
If this is a factor to be considered in a specific test, the solution should be completely aerated or deaerated in accordance with 8. The amount of possible contamination can be estimated from the loss g317-2 mass of the specimen, with proper application of the expected relationships among 1 the area of corroding surface, 2 the mass of g31-722 chemical product handled, and 3 the duration of contact of a unit of mass of the chemical product with the corroding surface.
For example, the accumulation of cupric ions in the testing of copper alloys in intermediate strengths of sulfuric acid will accelerate the corrosion of copper alloys, as g3-172 to the rates that would be obtained if the corrosion products were continually removed.
Cupric ions may also exhibit a passivating effect upon stainless steel coupons exposed at the same time. In practice, only alloys of the same general type should be exposed in the testing apparatus.
There are a number of other special types of phenomena of which one must be aware in the design and interpretation of corrosion tests. The behavior of the specimens in this galvanic couple are compared with that of insulated specimens exposed on the same holder and the galvanic effects noted.
It should be observed, however, that galvanic corrosion can be greatly affected by the area ratios of the respective metals, the distance between the metals and the resistivity of the electrolyte.
The coupling of corrosion coupons then yields only qualitative results, as a particular coupon reflects only the relationship between these two metals at the particular area ratio involved.
It is necessary to evaluate this localized corrosion separately from the overall mass loss. If agitation is required, the apparatus can be modified to accept a suitable stirring mechanism, such as a magnetic stirrer. A typical resin flask setup for this type test is shown in Fig. The suggested apparatus is basic and the apparatus is limited only by the judgment and ingenuity of the investigator.
For solutions corrosive to glass, suitable metallic or plastic kettles may be employed. A large surface-to-mass ratio and a small ratio of edge area to total area are desirable. These ratios can be achieved through the use of square or circular specimens of minimum thickness. Masking may also be used to achieve the desired area ratios but may cause crevice corrosion problems.
Circular specimens should preferably be cut from sheet and not bar stock, to minimize the exposed end grain. Special coupons for example, sections of welded tubing may be employed for specific purposes.
With wstm thickness of approximately 3 mm 0. The total surface area of a circular specimen is given by the following equation: This can be done by chemical treatment picklingelectrolytic g311-72, or by grinding with a coarse abrasive paper or cloth such as No. If clad alloy specimens are to be used, special attention must be given to ensure that excessive metal is not removed.
After final preparation of the specimen surface, the specimens should be stored in a desiccator until exposure, if they are not used immediately.
ASTM G_Laboratory Immersion Corrosion Testing of Metals – Free Download PDF
In special cases for example, for aluminum and certain copper alloysa minimum of 24 h storage in a desiccator ast recommended. The choice of a specific treatment must be considered atsm the basis of the alloy to be tested and the reasons for testing. A commercial surface may sometimes yield the most significant results. Too much surface preparation may remove segregated elements, surface contamination, and so forth, and therefore not be representative.
It may be desirable to test a surface representative of the material and metallurgical conditions used in practice. NOTE 1—The flask can be used as a versatile and convenient apparatus to conduct simple immersion tests. Configuration of top to flask t31-72 such that more sophisticated apparatus can be added as required by the specific test being conducted.
Occasional exceptions, in which a large difference is observed, can occur under conditions of borderline passivity of metals or alloys that depend on a passive film for their resistance to corrosion.
Therefore, at least duplicate specimens should normally be exposed in each test. The mechanical property commonly used for comparison is the tensile strength.
Measurement of percent elongation is a useful index of embrittlement. The procedures for determining these values are shown in detail in Test Methods E 8.
Determination of Metal Corrosion Rate by ASTM G31-72. Canadian Laboratory
If metallic contamination of the stamped area may influence the corrosion behavior, chemical cleaning must be employed to remove any traces of foreign particles from the surface of the coupon for example, by immersion of stainless steel coupons in dilute nitric acid following stamping with steel dies. However, the absence of cracking should not be interpreted as indicating resistance see 4. This resurfacing may cause some surface g31-772 hardening, to an extent which axtm be determined by the vigor of the surfacing operation, but is not ordinarily significant.
The surface finish to be encountered in service may be more appropriate for some testing. For relatively soft metals such as aluminum, magnesium, and copperscrubbing with abrasive powder is not always needed and can mar astj surface of ashm specimen. Proper ultrasonic procedures are an acceptable alternate. The use of towels for drying may introduce an error through contamination of the specimens with grease or lint.
If cleaning deposits for example, scouring powder remain or lack of complete dryness is suspected, then recleaning and drying is performed until a constant mass is attained. These factors include oxygen concentration, temperature, rate of flow, pH value, composition, and other important characteristics of asstm solution.
Molarity and normality are also helpful in defining the concentration of chemicals in some test solutions. Preferably, the use of a reflux condenser ordinarily precludes g31-2 necessity of adding to the original kettle charge. These changes should be determined if possible. Where required, the exhausted constituents should be added or a fresh solution provided during the course of the test. The actual time of contact of f31-72 metal with the solution must also be taken into account.
Any necessary distortion g31–72 the test conditions must be considered when interpreting the results. If clad alloys are exposed, special procedures will be required to ensure that only the cladding is exposed, unless the purpose is to test the ability of the cladding to protect cut edges in the test solution. Other procedures that require the removal of solid corrosion products between exposure periods will not measure accurately the normal changes of corrosion with time.
However, there are cases where this assumption is not valid. For example, lead exposed to sulfuric acid corrodes at an extremely high rate at first, while building a protective film; then the rates decrease considerably so that further corrosion is negligible.
The phenomenon of forming a protective film is xstm with many corrosion-resistant materials. Therefore, short tests on such materials would indicate a high corrosion rate and be completely misleading. With borderline conditions, a prolonged test may be needed to permit breakdown of astmm passive film and subsequent more rapid attack.
Consequently, tests run for long periods are considerably more realistic than those conducted for short durations. This statement must be qualified by stating that corrosion should not proceed to the point where the original specimen size or the exposed area is drastically reduced or where the metal is perforated.
Most tests related to process equipment should be run with the natural atmosphere inherent in the process, such as the vapors of the boiling liquid. Extraneous effects can be encountered if the air stream impinges on the specimens. A liquid atmospheric seal is required on the test vessel to prevent further contamination. For other degrees of aeration, the solution should be sparaged with air or synthetic mixtures of air or oxygen with an inert gas. Oxygen saturation is a function of the partial pressure of oxygen in the gas.
Methods of Cleaning Specimens after Test 9.