Q: I’ve heard that cobalt-base Alloy 6 materials should not be used in boiler feedwater service. Is this true?
A: Cobalt-chromium Alloy 6 is a very popular material for hard valve trim in applications requiring resistance to sliding wear, erosion and/or cavitation. It’s even successfully used in applications that are somewhat corrosive. However, in some areas of boiler feedwater applications where it would seem Alloy 6 should perform well, problems have been encountered. Here are some theories and facts about the problems:
Alloy 6 is available in cast, wrought and weld overlay forms. The cast material designation is UNS R30006; wrought material is designated UNS R30016, but is commonly called Alloy 6B; the generic AWS designation for hardsurfacing material is CoCr-A (specific designations are ECoCr-A for SMAW electrodes and ERCoCr-A for bare electrodes).
Alloy 6 is a cobalt-chromium-tungsten alloy with approximately 1% carbon. The material consists of a soft, solid solution matrix of cobalt-chromium tungsten surrounding a small percentage of hard, brittle chromium carbides.
Although the carbide phase provides the high hardness (approximately 40 HRC), research has shown that the cobalt-chromium-tungsten matrix is responsible for the alloy’s excellent wear and cavitation resistance. Alloy 6 undergoes a phase transformation (i.e., a change in crystal structure) when highly stressed, such as in a wear or cavitation situation. This phase transformation absorbs some energy that would otherwise cause damage, in effect, reducing the overall level of damage compared with a material that does not exhibit this “shock absorption” property.
Like most corrosion-resistant alloys containing chromium as an alloying element, Alloy 6 achieves corrosion resistance from the formation of a stable chromium oxide passive layer. This passive layer protects the underlying material from reacting with the environment. Certain chemicals can weaken the passive layer, reducing its ability to protect the material from corrosion.
Performance problems with Alloy 6 have been experienced in boiler feedwater applications where the water is treated with hydrazine or some other amine derivative. The problems occur exclusively in regions where the flow velocity is high, indicating that the failure mode is actually erosion-corrosion. Two possible explanations for this phenomenon are that:
1) The amine compounds weaken the oxide passive layer so that it erodes easily. The passive layer is repeatedly eroded away and rebuilt, resulting in accelerated corrosion.
2) The amines prevent the oxide passive layer from reforming after it has eroded initially, thus leaving the alloy unprotected from corrosion. Further erosion-corrosion then occurs at accelerated rates.
Other possible mechanisms for this type of failure may also exist, but the point remains that the alloy is attacked at higher rates than would be expected in an equivalent water application without the presence of the amine compounds.
Studies of returned parts have demonstrated that these attacks definitely correlate to the presence of Alloy 6. The photographs in Figures 1 through 4 were obtained during evaluation of a valve plug with Alloy 6 seat and guide surfaces that suffered erosion- corrosion damage in boiler feedwater. It’s very clear that the damage occurred exclusively in the CoCr-A material, and that the adjacent S31600 material is relatively unaffected, even though it is much softer.
Many failures have occurred in feedwater regulating valves, too. Failures have been reported in special- and standard-trim valves operating at temperatures as low as 300° F (149° C) and pressure drops as low as 100 psi (7 bar). Similar failures have been experienced in tungsten carbide trim in amine-treated feedwater and in ammonia applications when a cobalt binder phase is used.
No amine content, temperature or velocity limits have been established for safe use of Alloy 6 materials in feedwater. Common practice is to avoid the use of cobalt-containing alloys in feedwater service unless the feedwater is known to be compatible with cobalt alloys.
Common alternatives to Alloy 6 include hardened stainless steels such as S41600 (Type 416 SST), S41000 (Type 410 SST), S42000 (Type 420 SST), S44004 (Type 440C SST), or S17400 (17-4 PH SST). In some severely erosive applications, nickel base or iron-base hard-surfacing materials have been used.