Intergranular attack, evaluating

For both heat-affected zone corrosion (intergranular attack) and knifeline attack the heat flux during welding and the time at temperature can critically affect the severity of the attack. Both these factors may vary from one welder to another, and when preparing pieces for corrosion testing not only should fabrication welding conditions be accurately reproduced, but the work of more than one welder should be evaluated. 

Table 19.4 Maximum acceptable evaluation test rates specified by Du Pont for services where susceptible material would be intergranularly attacked ...

Electrolytic etching of stainless steels has been used industrially to simplify and accelerate the evaluation of stainless steels for their susceptibility to intergranular attack. Be-... 

Testing of Intergranular Attack This necessitates metallographic examination (30). The Standard Practice ASTM GUO (1992) involves evaluation of IGC resistance of heat-treatable aluminum alloys by immersion in a sodium chloride and... 

All of the alloys listed in Tables 1 and 2 may become sensitized that is, form various precipitates at grain boundaries when exposed to certain temperatures and thereby become subject to intergranular attack. Chromium-rich carbides are the most common precipitates in the alloys of Tables 1 and 2, except in the Ni-Cr-Mo alloys in which molybdenum carbide is formed. All of the evaluation tests discussed below detect susceptibility to intergranular attack associated with chromium and molybdenum carbide precipitates. 

Generally, evaluation in the 50 % sulfuric acid ferric sulfate test is by weight-loss. However, on alloys C and C-276 the relatively low chromium content of about 15 % (i.e., 3 % less than in Type 304 steel) results in a somewhat high rate of general corrosion that tends to mask low rates of intergranular attack. Therefore, to establish evidence of intergranular attack, microscopic examination is recommended in ASTM G 28 to supplement the corrosion rate (weight-loss) data. The newer C-type alloys (Table 1) have 20 to 23 % chromium, and therefore do not pose this problem. 

When available plant experience shows clearly that there will be intergranular or other localized attack on sensitized material, the alloy intended for such service must obviously be free of damaging grain-boundaiy precipitates and should, therefore, be resistant to intergranular attack in an appropriate evaluation test. 

Streicher, M. A., Theory and Application of Evaluation Tests for Detecting Susceptibility to Intergranular Attack in Stainless Steels and Related Alloys—Problems and Opportunities, Intergranular Corrosion of Stainless Alloys, ASTM STP 656, R. F. Steigerwald, Ed., ASTM International, West Conshohocken, PA, 1978, pp. 3-84. 

Evaluation of corrosive effects arising during the exposure can be evaluated by visual inspection, metaUographic examination of cross section under microscope with respect to pits, cracks, intergranular attack, and dezincification (of brass) and determination of loss in heat transmission capacity, a method which has direct relation to the function of automotive radiators. 

The accelerated Strauss test, ASTM A 262 Practice E (copper-copper sulfate-16 % sulfuric acid test), is also a popular quality control test because it is brief (typically 24 h) and includes an acceptance criterion. After exposure, specimens are bent to expose intergranular attack. Material with attack fails, while unattacked material passes. Experience is important in specimen evaluation to separate cracks" due to attacked grain boundaries from mechanical cracks that may have occurred in bending. Different laboratories have obtained varying results upon examination of the same set of sjjecimens [/2]. 

Evaluation of intergranular attack is more complex than evaluation of pitting. Visual observations are generally not reliable. For 5xxx series alloys, a weight-loss method has been accepted by ASTM (ASTM G 67). For heat treatable 2xxx and 7jdcc aluminum alloys, sus-... 

Evaluation of Results After the specimens have been reweighed, they should be examined carefully. LocaHzed attack such as pits, crevice corrosion, stress-acceleratedcorrosion, crackiug, or intergranular corrosion should be measured for depth and area affected. 

As is the case with other types of corrosion testing, mass-loss determinations may fail to indicate the actual damage suffered by specimens that are attacked intergranularly or in such a manner as dezincification. In such cases, mechanical tests will be required as discussed already in the section on evaluation techniques. 

There are a number of tests available for the evaluation of the sensitivity of a given stainless steel to intergranular corrosion (Table 7.41). They differ in severity of corrosion conditions and therefore do not reveal the same phenomena. The oxalic acid test consists of an electrochemical attack at constant current. The morphology of the corroded surface is then compared with reference samples. The main advantage of this technique is its rapidity (the attack takes about one minute). The applied potential lies in the transpassive region, where the chromium easily dissolves. The oxalic acid test therefore does not reveal the zones depleted in chromium, but rather exposes the presence of carbides and intermetallic phases. 

The most commonly-used steady state techniques are potentiodynamic tests to determine the corrosion rate in systems that experience a uniform corrosion process. This type of attack can also be studied by measuring resistance to polarization. Cyclical polarization ciurves are also used to study localized corrosion and potentiokinetic reactivation is the most suitable study technique for evaluating intergranular corrosion produced by a sensitization phenomenon following ASTM G108 standard test. 

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