Chromatic Changes
Recommendation
Chromatic interactions are a great indicator of the kinetic and chemical reaction throughout the course of a chemical cleaning.
It is of significant importance to collect samples from different part of the system during the treatment operation as color changes provide reliable information about the evolution of the reaction taking place.
Explanation
Inhibitive effects of sodium nitrite (among other passivation agents) are accepted due to the oxidation of Fe2+ ion to form a very thin layer (40-100 armstrongs) of gamma-Fe2O3 on the piping inner surface under alkaline solutions.
The color of this final compound is black or brownish-black and along with the redox potential measurement will determine the completion of the process.
The picture below displays from left to right color changes in all stages of an HF acid treatment methodology.
