Boiler tubing, corrosion damage

Under certain corrosive conditions, many metals form covering layers. If these are sufficiently dense, they act as protective films against the corrosive removal of the material. An example of this is the protective layer of iron oxide formed in unalloyed or low-alloy boiler tubes. Corrosion with erosion is understood as the combined action of mechanical surface removal and corrosion. With some soft and loose layers, the shear forces obtained with pure flowing liqnids at medium flow velocities are sufficient to damage the protective layer without the involvement of abrasive solid particles. Where drop impingement or cavitation is involved, the mechanical removal of material is understandable. 

The periodic development and use of new steel alloys can improve ferrous corrosion resistance however, where economizer units are constructed of copper alloys, under certain conditions serious copper corrosion problems may result. This occurs when FW having a pH over 8.3 also contains small amounts of ammonia and dissolved oxygen (DO). The ammonia may be present, for example, as a result of the overuse or inappropriate application of certain amines. Further damage may occur from the plating-out of the copper-ammonia ion then created as a cathode on boiler tubes. This promotes anodic corrosion of the immediate surrounding anodic areas. 

An alternate method of classification of damage mechanisms in terms of environmental conditions such as stress, temperature, corrosion, wear, radiation is known as the failure wheel. In this representation secondary mechanisms are underlined as opposed to primary damage mechanisms. The boiler tube failure discussed can be represented in terms of failure wheel, as shown in Figure 2.31. The aim in any case is to identify definitively the underlying mechanism of failure to enable one to undertake remedial action such that future failures do not occur. 

Reactions of this kind account in part for pitting and grooving of boiler tubes and are those that account for the excessive corrosion rate of iron at high values of pH, as Fig. 18.4 shows. As mentioned earlier, corrosive concentrations of NaOH are usually built up by evaporation of alkaline boiler water at various crevices where liquid flow is impeded and heat transfer is restricted. In the absence of conditions that favor concentration of alkaline solutes, damage by corrosion is expected to be minimal. Furthermore, phosphate additions avoid the high values of pH in concentrated boiler water that are necessary for reactions (18.10) and (18.11). 

Corrosion hydrogen monitor Boiler tubes, feedwater system, and PWR steam generator corrosion Detect general corrosion vs. load and chemistry hydrogen damage, caustic gouging in boiler tubes, potential for impurity concentration... 

Applications involving the reduction-to-practice of pulsed thermography techniques include an Electric Power Research Institute (EPRI) project to detect the extent of erosion/corrosion damage in power generation boiler tubes. The material in this section is reprinted with permission from EPRI. 

On occasion, failures that occur in boiler tubes can be attributed to an erosion mechanism. They generally occur at areas in the tubes where the normal direction of flow has been altered abruptly, a condition of turbulence created and a new flow path followed. The resultant corrosion is similar to that found in some feed-line systems. Here again a situation exists where the primary cause of the failure is a physical one, i.e. the flow pattern, while the resultant chemical corrosion causes the damage. 

In most WTE plants, steam conditions of 300-450°C/2.9-5.8 MPa as shown in Fig. 19.1 are adopted to avoid corrosion damage to boiler tubes and to ensure stable operation. Due to increasing enviromnental concerns arising... 

Corrosion of superheaters and water walls in waste fired boilers results from the deposition of fly ash on the metal surface. This produces a characteristic failure situation in which the covered part of the tube is significantly damaged (Fig. 19). In these ashes, molten phases are formed, mainly chlorides KCl, NaCl, ZnCl2, and PbCl2 on cooler parts such as water... 

It is possible also for the caustic to concentrate at the waterline. Generally, the waterline area is always most sensitive to corrosion. Two types of failures are common in boilers and both are related to the effect of concentration of caustic. One which forms discontinuous microcracks and results in the bursting of tubes is called hydrogen damage, and the other which results in the formation of continuous microcracks leading to intergranular corrosion is called caustic cracking. Both are briefly discussed below ... 

---