Typical bolt with cracks revealed using Liquid Penetrant Inspection
Magnetic Particle Inspection
Magnetic Particle Inspection can reveal surface flaws and near surface flaws.
Passivation is a process that removes embedded iron particles from the surface of stainless steel parts to prevent this from happening.
Chemical Film is a non-electrolytic process that deposits a chromate layer onto the surface of aluminum parts. This layer either provides protection for parts that are subsequently painted or makes the parts more electrically conductive.