Metal Mechanical Finishing


Metal Cleaning: Step-by-Step Guide

Metal cleaning is important when it comes to rebuilding and refurbishing parts and even whole machines. Nearly all industrial facilities have a robust preventative maintenance program designed to keep their equipment operating for years longer than they otherwise would.

In many places, thanks to their extensive maintenance programs, the same equipment has been running for decades. Part of those programs will involve cleaning old parts, removing old grease that has built up over the years, chemicals that might have coated the inside of pipes and valves, and of course, rust and other corrosion products. Metal Mechanical Finishing

In this article, we break down the process to cleaning metal.


The cleaning of metal objects to be processed produces various wastes. If degreasing is required, an important waste that must be managed is the waste solvent, as well as the substance removed. In general, the most appropriate way to manage the waste solvents is to regenerate for reuse, then dispose of the residuals. For instance, if a chlorinated solvent is used for degreasing, regeneration is accomplished by use of a still. The still bottoms are then treated and disposed of by one of the methods described in Chapter 7.


In general, rinsing after any of the five processing steps (cleaning, deoxidizing, etching, anodizing or alodizing, and coloring) produces wastes that are simply dilute forms of the wastes produced directly by those processing steps. Often, rinsing wastes are best managed by operating the rinsing process in a counter-current mode, where two or more rinsing baths are used for each of the processing steps. Clean makeup water is continually added to the final rinse tank, which overflows in the rinse tanks that precede it in the processing steps, and so on, if more than two rinse tanks are used. The overflow from the first rinse tank after each processing step, anodizing, for instance, is then used as makeup water for the process itself.

Deoxidizing and Etching

Deoxidizing and etching are both done with either caustic or acid solutions. Deoxidizing may use both, in series, if the metal to be anodized or alodized is badly oxidized. The purpose of deoxidizing is to remove oxides that have formed naturally but in an undesirable manner. The purpose of etching is to expose a clean, fresh metal surface for the anodizing or alodizing process. Wastes contain spent acid or caustic solutions and ions of the metal being coated.

Anodizing or Alodizing

The spent acid baths from the anodizing or alodizing process constitute the major waste from the coating process. These solutions must be maintained above a certain quality for the coating processes to be satisfactory. Build-up of metal salts (from the metal being coated) must be kept below a level where they interfere with the anodizing or alodizing process. Also, as the solution is weakened by drag-in from the previous rinse process, the active ingredients must be made up to maintain a required minimum concentration.

In continuous flow anodizing or alodizing operations, continuous makeup of water and active ingredients can maintain successful operation for a period of time; however, as the acid or alkali bath solutions continually attack the metal being coated, eventually the bath must be dumped and the process restarted with new anodizing or alodizing solution. These dumps of spent solutions, which normally occur about once per month, represent a major waste stream.

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