Identifying Probable Compound Using its Color & Reactants

What has this to do with hypochloritess, Boss?

Following is the source document. Basically the Caustic (besides Cl2) is the main raw material in industrial NaOCl manufacture

http://www.forceflow.com/hypochlorite/Hypo_Handbook_Oxy_Chem.pdf

Is it Zinc?

@ http://en.wikipedia.org/wiki/Black_oxide

According to WIKI my 'probable compound' rather is a mixture!

Hot Black Oxide for Stainless Steel is a mixture of caustic, oxidizing, and sulfur salts. It blackens 300 and 400 series, and the precipitation hardened 17-4 PH stainless steel alloys. The versatile solution can be used on cast iron and mild low carbon steel. The resulting finish complies with military specification MIL-DTL-13924D Class 4 and offers abrasion resistance.

Patina on copper does the same for the Statue of Liberty

The black film is Fe(II). If it is allowed to dry and then exposed to water below 1% NaOH concentration (I believe this is the magic number but open to correction- pH of 12.5-13.0 or above), especially if not actually submerged, it is liable to form a hydrated Fe (III) complex- rust. It may be more of a burnt brown than the typical red due to partial oxidation to Fe (III) throughout the film.

firstly: poor transfer of information about the precise metallurgy will result in GIGO response (verbal upchuck related to conjecture). If on a carbon steel we are talking magnetite, but on stainless steels, the oxide on the surface will be complicated by the presence of chromium, and possibly molybdenum. Be careful what you ask for. Why are you not using a plastic vessel?

One issue in caustic soda storage in carbon steel vessels is the stripping of oxides from the surface when ferrite ions are formed (FeO2-), which hydrate well and are quite stable in solution. Perhaps you are only referring to passivation of vessels to be utilized in transport of caustic soda, and perhaps an oxidizer would be useful in such a case.