Au999has excellent chemical stability

however

the products made of Au999 sometimes form‘rusty spots’.Rusty spots of Au999 pure gold bars were investigated by scanning electron microscope（SEM）

micro-area energy dispersive spectrometer

XRF

chemical analysis

XPS and field investigation.The results indicate that the overall gold content of pure gold bars meets the standard requirements.However

the surface of the‘rusty spots’ area has amounts of Ag

S and O elements and trace of Cl

Si and Na elements.Ag and S exists in form of Ag

2

S

which is responsible for tarnish.Meanwhile

Ag

2

S and trace of SiO

2

are corroded and consumed after acid treatment

resulting in micro cavities on the ‘rusty spots’ area.These impurity elements are caused by improper production management.Thus

to solve the‘rusty spots’ of pure gold bars

it must improve management of the production site and optimize the production process and technical process as well as keeping the worksite clean and strengthening the inspection and control procedure.