It resists flowing seawater, brackish water and marine atmosphere far better than stainless steel, and it does not suffer chloride stress corrosion cracking, which is the failure mode that ends austenitic stainless components in the same service. Pump and propeller shafts, valve trim, seawater piping and fasteners are typical.

Alloy 400 is solid-solution strengthened, ductile and tough, and cannot be hardened by heat treatment. K-500 contains aluminium and titanium so it can be age hardened to roughly three times the yield strength while retaining most of the corrosion resistance, which is why shafts, stems, springs and fasteners specify it.

Because very few metals tolerate HF at all, and Monel does across a useful range of concentration and temperature. It is effectively the default material of construction for HF alkylation units and fluorine chemistry, covering piping, vessels and valve components where the alternative would be a lined or exotic material.

Oxidising conditions. Nitric acid, strong oxidising salts such as ferric and cupric chloride, and hot concentrated sulphuric all attack it, and ammonia in the presence of moisture and oxygen can cause stress corrosion cracking. It is a reducing-condition and chloride alloy, not a universal corrosion-resistant one.

Alloy 400 is weakly magnetic at room temperature with a Curie point close to ambient, so its magnetic response varies with temperature and cold work and it should not be treated as reliably non-magnetic. K-500 is essentially non-magnetic in the aged condition, which is sometimes the reason it is specified for instrument components.

Slowly. It work-hardens rapidly, is gummy and tends to build up on the tool edge, so it requires sharp, rigid tooling, positive rake, generous feeds that cut beneath the hardened layer and moderate speeds with good coolant. Age-hardened K-500 is more abrasive but cuts more cleanly than the softer alloy 400.

It does, and that is a genuine strength. It keeps its toughness and ductility down to cryogenic temperatures with no ductile-to-brittle transition, which is why it appears in cryogenic handling equipment and in low-temperature valve and fastener applications alongside its more familiar marine use.