Ductile materials can absorb large amounts of energy without fracturing. Copper and gold can be beaten and pressed into nearly all forms without fracturing. Hard materials, the opposite of ductile, on the other hand do not absorb energy without cracking and fracture. High stress in hard materials will initiate microcracks, which makes it easy for H2S to enter the iron structure and begin forming iron sulfide corrosion products and hydrogen. The hydrogen gets diffused into the metal furthering the cracking process. If softer materials are used, there is less crack formation, making it more difficult for the process to begin, There is a more accurate description here...
Petroleum folks believe placing a max hardness requirement helps to prevent SCC/HSC, particularly in H2S environment, and this does make sense in some perspectives, but may not make fully sense metallurgically (no correlation between hardness and corrosion resistance). One can find lots of debates on that. A material supplier can be seen often to try to stay away from NACE hardness req by any means.
There’s no great correlation between hardness and electrochemical corrosion, but the cracking mechanism isn’t electrochemical. The hardness is a coarse parameter indicative of a susceptible microstructure, as is the 1% nickel limitation as that is believed to be a coarse guide to a carbon/low alloy steel having a susceptible microstructure. A large number of suppliers see “NACE Requirements” as an easy way to fleece the purchaser.
Hardness is very easy to obtain. It gives a rough idea of the strength and microstructure of the material. To help end users save effort, time and money, standards specify hardness.
