Welds corrosion testing

Selective corrosion in the heat-affected zone of a weld occurs most commonly when unstabilised stainless steels are used in certain environments. The obvious answer is to use an extra-low-carbon grade of stainless steel, e.g. types 304L, 316L or a stabilised grade of steel, e.g. types 321 and 347. Knifeline attack at the edge of a weld is not commonly encountered and is seldom predictable, and it must be hoped that it is revealed during preliminary corrosion testing. 

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. 

Corrosion Tests of Flame Sprayed Coated Steel-19 Year Report, American Welding Society. Miami (1974)... 

Welding results in a metallurgical discontinuity with different microstructures than the parent metal. Mechanical properties of the weld metal usually differ. Even with PWHT, weldments retain a residual stress field. Heat affected zones often contain a coarsened grain structure and/or hard spots. Corrosion testing of the welds may be a critical part of the testing program. 

Schedule 40, 347 SS, 4-inch pipe, a 500-gallon-per-minute sodium pump, a corrosion test section, and a 35-gallon expansion tank. Each and every part was electropolished, pipe ends were taped dust-tight before welding, inert arc gas back welds were made, and every effort was made to complete, in the field, a chemically clean sodium system. Figure 22 shows the dirty sodium surface which resulted after clean sodium was filled into the loop. This experience demonstrated very conclusively the importance of being practical in sodium system construction. 

Scientists (Ref 12) welded HSLA-65 using tungsten-base tools. Subjected to bend tests, a 10 mm (0.4 in.) thick weld passed, and a 6 mm (0.24 in.) thick weld failed when bent with the root in tension, due to the formation of surface cracks. Tensile properties of the 10 mm thick welds exceeded the specifications for the base metal. Some 6 mm thick welds exceeded the plate specifications, while others were approximately 10% below the plate specifications. Charpy V-notch (CVN) toughness at both -29 and 0 °C (-20 and 0 °F) were below the base material toughness but exceeded the minimum specification of the plate. The surface of the welded material was found to have small defects due to the roughness caused by the interaction between the shoulder and the surface of the plate. Salt spray corrosion tests indicated no preference for corrosion in the weld zone. 

A typical rack employed for installation of specimens in pilot plants is shown in Fig. 6. Both corrosion coupons, 2 X 1 X 0.35 in. thick, and bend specimens intended to determine stress-corrosion cracking susceptibility, are included in the installation for aqueous corrosion testing. Specimens are separated by high density alumina spacers to eliminate electrochemical effects. During exposure, the racks are welded to existing components in the pilot plant equipment. 

Corrosion tests in Flame-sprayed Coated Steel. 19 Years Report. Miami American Welding Society, 1974. 

ASTM G 58, Practice for Preparation of Stress Corrosion Test Specimens for Weldments— This covers the manufacture and application of welded test specimens in stress corrosion testing. This practice is used to evaluate a total weldment, weld metal, or presence of notches and stress risers in weldments with respect to SCC in an environment. In addition, ASTM G 58 contains a method for evaluating the critical stress levels that will produce SCC in a weldment. The specimens are evaluated after exposure to an environment by microscopic methods. This standard includes typical specimens (stressed and tension), welding considerations, test specimen preparation, and inspection after exposure. 

Corrosion test specimens are used to evaluate average corrosion rate over the exposure period and eire also useful for assessment of crevice corrosion, pitting, and end grain attack, and may be used for metallographic examination of the corrosion test specimen or analysis of any deposits. Special corrosion test specimens may be prepared with welds to assess corrosion problems particular to weld material or heat affected zones. SCC may be monitored with specially mounted and loaded corrosion test specimens. 

ASTM G 58, Practice for the Prepeuation of Stress Corrosion Test Specimens for Weldments, describes use of aU of the before-mentioned smooth test specimen configurations. It also includes some configurations unique to welding practices, and references citing use of specimens and procedures unique to SCC testing of weldments. 

Out-of-pile stress corrosion tests been mounted on plate and weld sections of N4 v hich had been previously exposed In HERALD reactor for periods up to fa months In boric acid solutions. No failures have so far been observed. 

Figure 28 Comparison of the predicted and observed crack growth rates vs. solution conductivity for in-plant GE CAVS data on alloy 182 weld metal tested at constant load in BWR water of varying chemistry (corrosion potential).

Preparation of test pieces of welded metal for stress corrosion testing... 

These examples demonstrate the necessity for testing each galvanic couple in the environment for which it is intended. Higher-aUoy filler metals can sometimes be used to advantage to prevent rapid preferential weld corrosion. 

Grain growth and the formation of embritthng phases are two other negative effects of high heat inputs. When there is uncertainty regarding the effect that welding conditions wiU have on corrosion performance and mechanical properties, a corrosion test is advisable. 

Examination of equipment fabricated from tantalum that has been used in a wide variety of service conditions and environments generally shows that the weld, HAZ, and base metal display equal resistance to corrosion. This same resistance has also been demonstrated in laboratory corrosion tests conducted in a number of different acids and other environments. However, in applications for tantalum-lined equipment, contamination of the tantalum with iron from underlying backing material, usually carbon steel, can severely impair the corrosion resistance of tantalum. About the only known reagents that rapidly attack tantalum are fluorine, HP and acidic solutions containing fluoride, fuming sulfuric acid (H2SO4) (oleum), which contains free sulfur trioxide (SO3), and alkaline solutions. 

Lastly, dissimilar weld filler metals can be successfully used. To avoid premature failure the dissimilar combination shonld be corrosion tested to ensure suitability for the intended service. 

Figure 12.11 Preferentiai attack of the continuous austenite phase in an autogenous GTA weld in Ferral-ium Alloy 255. Crevice corrosion test was performed in synthetic seawater according to ASTM D 1141 at 100°C (212 °F). Etched with 50% FINO (xt 00). (Reproduced with permission from Daubert Cromwell.)...

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