# Technological Processes for Steel Applications in Nuclear Fusion
## 6 Solid State Bonding
### Corrosion and Contamination  
Opposed to laboratory conditions, industrial fabrication procedures are usually far from perfect with respect to cleanliness and chronology of the production steps. Both are oriented towards high efficiency and easy application. Since for diffusion bonding clean surfaces are obligatory, there might be a discrepancy between optimum joining quality and commercial fabricability of large parts such as blankets. Surface contamination includes two basically different effects, which may lead to and act as diffusion barriers, and thereby, deterioration of the weld properties. Evidently, one is oxidation. This applies especially to chromium steels, where the corrosion resistance is caused by thin Cr\({}_{2}\)O\({}_{3}\) surface layers, which in turn suppress diffusion significantly. The other effect literally results from \"dirt\". In industry, this typically results from contact with oil, coolants, dust, water, wrappings, and many more contaminants. That is, prior to diffusion bonding, the surfaces have to be cleaned. A problem arises here, since there might still remain traces of the cleaning agents or of the dirt or of both. In any case, such remnants could react locally with the surface before and during the HIP process, and they might also act as a diffusion barrier. Figure 27: (**a**) Micrograph of the cross-section from the sample with surface machining # 2 after diffusion bonding by one HIP cycle at 25 MPa/1050 \({}^{\circ}\)C/2 h. In this case, the weld surface (indicated by the black arrows) is still recognizable; (**b**) The same cross-section after a second HIP cycle at 100 MPa/1050 \({}^{\circ}\)C/2 h. The weld surface cannot be identified anymore. It is well known that 9%-Cr-steels such as EUROFER rust quickly in (sea-)water, in acids, or on contact with sweaty fingers. But the interesting question in connection with diffusion bonding is the rate at which an oxidation layer forms on a fresh fabricated surface in a common (industrial) environment. As previously demonstrated, the perfect final surface processing step consists in dry milling. Then, before the HIP process, both surfaces have to be fixed and joined together by a circumferential and vacuum tight electron beam weld line. During industrial production, there will always be a more or less extended period between both steps, and this means that more or less extended oxide layers will form. This effect was studied by a first test series (see Table 5, ID T0-T4). The reference sample T0 was fabricated similar to the previously described surface procedure # 4. It was immediately sealed vacuum tight by an EB-weld after milling. Four other samples have been put in a humidor with a constant relative humidity of 70% for 24 h, 48 h, 72 h, and for 14 days (T1-T4) prior to sealing in the EB-weld chamber. Finally, the samples were put in a HIP for two hours at a pressure of 25 MPa and at a temperature of 1150 \({}^{\circ}\)C. We increased the HIP temperature, because it is well known that diffusion bonding in the range around 1050 \({}^{\circ}\)C is sensitive to oxide layers. At higher temperatures, these might dissolve. Therefore, we applied only a low pressure, to allow strong effects of the treatments. Thereafter, the standard post-weld heat treatment has been applied before fabrication and testing of the Charyp specimens. The results are plotted in Figure 28. First of all, there are two clear outliers in the reference series at \(-\)80 \({}^{\circ}\)C, which otherwise comes rather close to the base material results. Further, it is remarkable that all oxidation conditions led to the same upper shelf energies and the DBTT is shifted by only 30 K, in the worst case. Nonetheless, there is an effect from different oxidation periods: It seems that 24 h oxide formation does not deteriorate the diffusion bond considerably. After 48 h, however, there is a deterioration of DBTT, but which is not further impaired by extending oxidation periods up to 2 weeks.