... the sample is placed into a copper sulfate solution. Any remaining iron will show up as pink spots on the sheet. This would be considered unacceptable
Passivation:
Passivation describes the treating of a metal with a mild oxidant (such as nitric acid) to remove surface iron or iron compounds by dissolution. This action forms a protective passive film on the surface of the metal. The trace iron left behind from machining and fabrication can provide sites for corrosion if left untreated.
The process is performed by first cleaning the surface with solvents or an alkaline solution to remove organic or metallic residues. Next, the parts are placed into passivation solutions. The choice for this solution is usually nitric acid. The process is controlled by three key variables: time, temperature, and concentration.
More information on this and other chemical engineering questions:
http://www.cheresources.com/archivezz.shtml (http://www.cheresources.com/archivezz.shtml)
http://www.normen.net/Stahlschluessel-2001-Deutsch-Franzoesisch/3922599176.html (http://www.normen.net/Stahlschluessel-2001-Deutsch-Franzoesisch/3922599176.html)