A: MR0175 (now ISO 15156) deals with sour service cracking (sulfide stress cracking) in H₂S environments. SP0170 deals with polythionic acid cracking during shutdowns. They are complementary but distinct.
This is the primary source.
Cathodic protection is a technique used to prevent corrosion on metal surfaces by applying an electric current to drive the corrosion reaction in the opposite direction. This is achieved by installing an anode, which is a sacrificial metal that corrodes in place of the protected metal. The anode is connected to a power source, which drives the electric current through the electrolyte (usually soil or water) to the protected metal surface. nace sp0170 pdf