Metal surfaces must be cleaned during post-metal treatment operations, before a finish can be successfully applied. Prior to painting or any other metal surface preparation, all foreign matter must be removed for acceptable adhesion to occur, and a smooth coat. It is also necessary that bare metal and even some coated surfaces are cleaned from time to time in order to maintain their integrity and prevent damage due to corrosion. In different environments, a variety of stuff can be deposited onto surfaces.
Types of deposits
Types of foreign matter may include:
- Stencil markings
- Corrosion oxides
Deposits on the metal surfaces can be simple or complex mixtures of inorganic and organic substances.
Chemical Bathing Inorganic Deposits
- corrosion products
- oxides, hydroxides and carbonates of Fe, Mg, Ca, Zn, Cu and Mn. For example, these can be geothite FeOOH and haematite Fe2O3
- sulphates of Mg and Ca
- phosphates of Fe, Mg and Ca. (hydroxyapatite (Ca3(PO4)2; Ca(OH)2)
- sulphides of Zn, Cu, and Fe
- silicates (acmite Na2O; Fe2O3; 4SiO2)
Chemical Bath to Remove Organic Deposits
- mineral and animal oils and fats
- algae and slime found in cooling water circuits
- heat denatured product
- polymers and hydrocarbons formed in various process streams
The deposits on metal surfaces can exist in a range of forms, from soft, light, easily removable types to complex, hard, layered types. Corrosion resistant pipe fitting coatings and temporary rust preventatives also must be removed prior to commissioning plant. This poses additional problems, because such coatings are often designed to be chemically resistant.
Cleaning & Stripping Metal with Chemical Bath
A combination of chemical reaction, detergency, mechanical action and solvent dissolution helps achieving effective surface cleaning. There are various cleaning methods for metal, divided in mechanical cleaning methods and chemical methods. The main methods employed in cleaning metal parts include soak, electrolytic and spray.
Chemical baths for cleaning of metals come with a number of advantages. The greatest advantage is that the metal surfaces are easy to be cleaned. If they are part of complex items or equipment, they do not need to be first dismantled and reassembled after chemical cleaning took place. The cleaning doesn’t leave areas more vulnerable to corrosion than before and doesn’t damage the equipment.
Metal surfaces get typically both inorganic and organic substances deposited on it. These substances are coming from corroding metal, various lubricating oils, deposits from hard water, deposited product, etc. Typically, these substances are removed in a five step process. However, this process may vary depending on the exact nature of the deposits.
Chemical cleaning primarily uses chemical solutions in order to remove deposits from metal surfaces. The process that involves circulation of chemical cleaning solutions without dismantling existing metal equipment is called cleaning in place (CIP). For cleaning stainless steel dairy equipment are extensively used nitric acid (HNO3) and caustic soda (NaOH). The more diverse applications of cleaning chemistry are found in the heavy industry.
The cleaning processes of metal surfaces include hot alkaline detergent cleaning, solvent cleaning, acid etch, and electrocleaning. The waterbreak test is the most common industrial test for metal cleanliness.
This test involves thoroughly rinsing of the metal surface that is then held vertical. Oils and other hydrophobic contaminants allow the water to rapidly drain, because it beads and breaks up. Perfectly clean metal surfaces will retain an unbroken sheet of water that does not drain of or bead up because they are hydrophilic.
Where to do Chemical Bathing
The place of metal cleaning by chemical bath is another aspect to take into consideration. At The Powder Coating Factory, we do chemical bathing on site. This is typically in case of maintenance and repair work, or prepping metals for powder coating.