When a study of the synthetic urea process  was undertaken at this laboratory, it soon became evident that the reacting system would attack the materials used in ordinary plant construction and  result in serious operating difficulties. This was especially true for such parts of the apparatus as the discharge valve of  the autoclave, which had to be closely machined in order to function properly. This valve was subjected, not only to both corrosion and erosion by the molten reaction mixture consisting of urea, water, and ammonium carbamate, but also to a pressure of 100 atmospheres and  a temperature of about 150 oC. It was thus apparent that the requirements for the metal used in the apparatus were most exacting.
The metal must be resistant to the corrosion and erosion of the melt; it must be capable of being machined; and it must be strong and tough enough to withstand the pressures and temperatures used, This paper will describe the results of an investigation to determine which commercial corrosion resistant alloys would best meet the exacting demands of actual working conditions.
Thirty-six different metal samples were studied in two tests, one of 4 days and another of 7 days duration. In each case, the samples were subjected to the corrosive action of the molten  reaction  mixture under pressure and at  142 oC, The first test was designed to include a large number of samples whose compositions made them appear suitable. The heat treatment which these samples received in their manufacture was not known. The second and longer run tested the best samples from the 4-day run and also a few metals such as silver, lead, nickel, and tin, which were known to be fairly resistant to corrosion.

1930 Thompson Bureau of Chemistry and Soil Resistance of Metals to Carbamate.pdf