Completely rust immune, great for use in salt-water conditions for dive knives and other water conditions. H1 is a PH steel, meaning it is a precipitation-processed alloy, that's rolled hard without heat-treating and has .1% nitrogen which acts like carbon, but does not react to chlorine to make rust. The edge can get up to 63-68 Rc and according to sources not become brittle. However, don't let the high Rockwell misled you, most consider the edge holding to be just under that of 8Cr13Mov or AUS-8.
** from Spyderco "We don't publish Rockwell but I can tell you that it is in the high 50's , low 60's and all of our micro hardness testing shows that it is harder at the edge than at the spine because it is differentially work hardened so in a serrated version, the performance is much higher than a plain edged version."
Carbon (C) 0.15%, Chromium (Cr) 14.00-16.00%, Manganese (Mn) 2.00%, Molybdenum (Mo) 0.50-1.50%, Nickel (Ni) 6.00-8.00%, Nitrogen (N) 0.10%, Phosphorus (P) 0.04%, Silicon (Si), 3.00-4.50%, Sulphur (S) 0.03%.