1) There are black spots on the black surface.
Fastener surface is not clean, maybe something dirty  on the surface.
        Fastener black is filled too much , which close to each other, so that the black liquor can not be uniformly touch to its surface.
        The oxide on the surface of the fast-formed fastener is not completely removed.

2) Oxide film on the thread become redness, which looks like rust.
        The proportion of black liquor improper.
        Beforeblack finish treatment, there is floating rust on the fastener surface.
        The surface contains phosphating residues.

3) Week oxide film adhesion,easy to fall off.
fastener surface cleaning is not complete, there are traces of oil.
        Black liquor temperature is too high.
        Corrosion of the fastener surface cleaning is not complete.
        Black time is too long, so that the fastener surface oxide film is too thick to fall off.

4) Thread fastener surface After black finish treatment,there is something on the surface,which influence the screw coupling.
        After tempering oxide, there is oxide skin.
        Mesh belt furnace knot a large number of black liquor condensate in the workpiece in the thread on the surface.
       There are foreign body in the black liquid.

5) The surface rust in the short term after black finish.
        Fasteners do not dry in time after drying or drying is not completely coated before anti-rust oil coating. Then surfacewill be with water molecules.
        Fastener surface black uneven, there are floating rust makes corrosion.
       There is water in the Long-term use of anti-rust oil tank, which should be promptly excluded.