Detecting Preferential Corrosion
By: Jose Aparicio - Director of Technical Services Co-founder at MetallurgyNDT, LLC
Detecting Preferential Corrosion
During the welding process, a number of important changes occur that can significantly affect the corrosion behavior of the weldment. Heat input and welder technique play important roles. The way in which the weld solidifies is important to understanding how weldments may behave in corrosive environments.
A metallographic study has shown that welds solidify into various regions, as illustrated in Figure 1. The composite region (1), or weld nugget, consists or essentially filler metal that has been diluted with material melted from the surrounding base metal. Next to the composite region is the unmixed zone (2), a zone of base metal that melted and solidified during welding without experiencing mechanical mixing with the filler metal.
The weld interface (3) is the surface bounding the region within which complete melting was experienced during welding, and is evidenced by the presence of a cast structure. Beyond the weld interface is the partially melted zone (4), which is a region of the base metal within which the proportion melted ranges from 0 to 100%. Lastly, the true heat-affected zone (HAZ) (5) is that portion of the base metal within which microstructural change has occurred in the absence of melting.
Although the various regions of a weldment shown in Figure 1 are for a single-pass weld, similar solidification patterns and compositional differences can be expected to occur in underlying weld beads during multi-pass applications.
Corrosion of Carbon Steel Weldments
The corrosion behavior of carbon steel weldments is dependent on a number of factors. Consideration must be given to the compositional effects of the base metal and welding consumable and to the different welding processes used. Because carbon steels undergo metallurgical transformations across the weld and HAZ, micro-structures and morphologies become important. Figure 2 is showing Internal side of an 8" pipe severely affected by preferential corrosion.
A wide range of micro-structures can be developed, based on cooling rates, and these micro-structures are dependent on energy input, preheat, metal thickness (heat-sink effects), weld bead size, and reheating effects due to multi-pass welding.
As a result of their different chemical compositions and weld inclusions (oxides and sulfides), weld metal micro-structures are usually significantly different from those of the HAZ and base metal. Similarly, corrosion behavior can also vary.
In addition. hardness levels will be lowest for high heat inputs, such as those produced by submerged arc weldments, and will be highest for low-energy weldments (with faster cooling rates) made by shielded metal arc processes. Depending on the welding conditions, weld metal micro-structures generally tend to be fine grained with basic flux, and somewhat coarser with acid or rutile (TiO2,) flux compositions.
During welding, the base metal, HAZ, and underlying weld passes experience stresses due to thermal expansion and contraction. Upon solidification, rather high levels of residual stress remain as a result of weld shrinkage. Stress concentration effects as a result of geometrical discontinuities, such as weld reinforcement and lack of full weld penetration (dangerous because of the likelihood of crevice corrosion and the possibility of fatigue cracking), are also important because of the possibility of SCC.
Achieving full weld penetration, minimizing excessive weld reinforcement through control of the welding process or technique, and grinding (a costly method) can be effective in minimizing these geometric effects. A stress-relieving heat treatment is effective in reducing internal weld shrinkage stress and metal hardness to safe levels in most cases.
Preferential Heat-Affected Zone Corrosion
This phenomenon has been observed in a wide range or aqueous environments, the common link being that the environments are fairly high in conductivity, while attack has usually, but not invariably, occurred at pH values below about 7 to 8. Figure 3 is showing axial weld cut with damage being detected by PAUT and TOFD in a 8 inch pipe circuit.
The reasons for localized weldment attack have not been fully defined. There is clearly a microstructural dependence, and studies on HAZs show corrosion to be appreciably more severe when the material composition and welding are such that hardened structures are formed.
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It has been known for many years that hardened steel may corrode more rapidly in acid conditions than fully tempered material, apparently because local micro-cathodes on the metal surface stimulate the cathodic hydrogen evolution reaction.
On this basis, water treatments ensuring alkaline conditions should be less likely to induce HAZ corrosion, but even at pH values near 8, hydrogen ion (H+) reduction can account for about 20% of the total corrosion current: pH values substantially above this level would be needed to suppress the effect completely.
Preferential Weld Corrosion
It is probable that similar microstructural considerations also apply to the preferential corrosion of weld metal, but in this case, the situation is further complicated by the presence of de-oxidation products, their type and number depending largely on the flux system employed. Figure 4 is showing inspected 8" pipe circuit with several welds being affected by preferential corrosion.?
Consumable type plays a major role in determining weld metal corrosion rate, and the highest rates of metal loss are normally associated with shielded metal arc electrodes using a basic coating. In seawater, for example, the corrosion rate for a weld made using a basic-coated consumable may be three times as high as for weld metal from a rutile-coated consumable.
Fewer data are available for submerged arc weld metals, but it would appear that they are intermediate between basic and rutile shielded metal arc electrodes and that a corrosion rate above that of the base steel can be expected.
Damage Mechanisms Inducing Preferential Corrosion and Affecting Fixed Equipment in the Industry.
There are several damage mechanisms that induce preferential corrosion affecting fixed equipment in the industry, such as, but not limited to: Carbon dioxide (CO2) corrosion, Amine corrosion and Hydrofluoric (HF) Acid Corrosion.
Nonintrusive Inspection for the Detection of Preferential Corrosion
UT would be the best inspection approaches for this damage without having a pipe size limitation. Figure 5 is showing PAUT and TOFD scan plan. PAUT and TOFD have a variety of probe size, crystals and frequencies to cover from very small pipe size to bigger components. Figure 6 shows PAUT and TOFD imaging resulting on damaged pipe weld.
PAUT can be set to detect and monitor HAZ, base metal and weld thickness and it would cover one or limited access welds. PAUT can detect and quantify smooth transitioned damage in HAZ and Base metal as well.
TOFD can help for sizing (depth and length) detection, as well as to prove up and confirmation of the damage. TOFD would be limited on very thin walls and access from both sides.
Both techniques can be deployed separately over the same weld and results can be compared during the analysis. These techniques need high experienced techs for data interpretation and for troubleshooting data acquisition. Figure 7 shows PAUT Imaging with preferential corrosion indication free from a 8" pipe weld.
References:
AMPP / NACE CIP level III Senior coatings inspection #4796
2 年José, thanks for such an interesting article. Best Regards.