described previously.' Portions 9f the coated loop that were accessible to a borescope were visually examined. The entire loop was radiographed. We determined that all surfaces of the loop were coated, but that several small cracks occurred in the coating at the corners of two joints where tubing was welded together at approximately a 90" angle. Also, the coating was thinner in one cross member than elsewhere, and a 'h-in.-diam blister occurred in the cross member near one of the joints mentioned above. Subsequently, we coated the two joints and the cross member again. Radiography revealed that the cracks on the joints were filled, the blister was covered, and the coating thickness in the cross member was comparable to other parts of the loop; however, a new crack was found in the cross member. This crack was probably caused by the large difference in thermal expansion between tungsten and Hastelloy N. The cross member was coated again, but the crack persisted, and further attempts to coat the loop have not been made. Several specimens of types 304 and 430 stainless steel were nickel plated by the electroless method so that the adherence of tungsten and molybdenum coatings to the specimens could be evaluated. The plating method used for these specimens yielded a phosphorus content of about 2% in the nickel plate compared with about 8% phosphorus in specimens plated previously. One specimen of each type steel was coated with about 0.OOCin.-thick tungsten. The coating was smooth in both cases but spalled from the type 430 specimen during bend testing. Another' pair of specimens was coated with about 0.001- to 0.004-in.-thick mo-
l l . J. I. Federer, MSR Program Semiannu. Progr. Rep. Feb.
28,1971,0RNL4676, pp. 231-32.
lybdenum. The coating on the type 304 specimen was too rough for accurate thickness measurements. The coating on the type 430 specimen was much smoother but spalled during bend testing. Since previous experiments showed that molybdenum coats smoothly on specimens electroplated with nickel, we believe that the electroless nickel was responsible for the poor coating characteristics. These results and the previous observation' that both tungsten and molybdenum coatings are less adherent to electroless nickel containing about 8% phosphorus indicate that electroless nickel is inferior to electroplated nickel for promoting adherence.
14.7 MOLYBDENUM DEPOSITION FROM M o F ~ J. W. Koger
We have continued to conduct experiments to optimize the conditions for coating stainless steel with molybdenum by contacting the stainless steel with a molten fluoride salt containing MoF6. Holding temperature and time constant, we have determined the concentration of Mop6 which must be added to our salt mixture to get a coating. Future plans involve an evaluation of time and temperature effects on coating rates and coating integrity. In one of our more successful .runs, microprobe analysis' of the type 316 stainless steel capsule on which molybdenum was deposited showed that molybdenum was deposited at the sample surface and that grain boundaries at least 350 I./ into the metal were enriched in molybdenum.
12. Performed by H. Mateer, T. J. Henson. and R. S. Crouse of the Metals and Ceramics Division.