Engineering Consulting
Surface Metal Cleaning
Problem Solving Made Easy
Stainless Steels are very resistant to corrosion; however, in some applications or harsh environmental conditions, some damages happen to the surface. Oxidation, corrosion and rusting can occur over the long run on the metal surface.
To protect the surface metal and achieve maximum corrosion resistance, we must keep the surface of the chrome steel clean. The chrome steel surface must be clean from contaminants physically stuck on the surface (oil, grease, and dust) or resulting from a chemical reaction (oxides, sulphides).
Different methods exist to remove dirt and oxide from the surface:
- mechanical and/or chemical methods,
- abrasive blasting, wire brushing or pickling and etching.
This paper will review the chemicals methods to treat the chrome steel surface and assure protection resistance. There are some chemicals methods to clean/treat the surface metal.
Classical methods are:
1- Surface decontamination using alkaline soaps, acid solutions or solvent immersion with mechanical or sonication.
2- Pickling remove oxide from the metal surface
3- Electro-polish: reduce micro-roughness and thickness of the steel.
4- Passivation: formation of a passive layer as a protective layer.
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Decontamination- Surface cleaning
The cleaning of oils and greases depends on their nature, if they're soap forming or not. The soap-forming oils and greases are those of animal or vegetable origin; the mineral oils don't form soaps.
The soap-forming oils and greases are been removed from the surface by transforming them by hydrolysis in fatty acids and by reacting these acids with alkaline solutions to get water soluble soaps.
The mineral oils are been removed by dissolving there in organic solvents, and, particularly cases only; they will be washed with alkaline solutions containing detergents.
Since the nature of the contaminants is usually unknown, a reliable cleaning must comprises two successive steps:
a) Degreasing with, organic solvents followed by
b) An alkaline degreasing.
The sequence of the cleaning operations begins generally with mechanical cleaning, followed by pickling, detergent cleaning and degreasing. to wash scale, rust, an abrasive blasting, or a wire brushing is typically used.
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Pickling
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The pickling process is a chemical process removing oxides, contaminants from the surface of a metal.
Depending on the concentration of the pickling solution and the pickling time operation, the cleaned surface of the metal may appear rough or smooth.
For an effective pickling process, the surface of the metal must be free of organic contaminations and the temperature of the solution needs to be controlled.
An increase in temperature will increase the pickling rate. There are, however, upper-temperature limits that need to be considered.
- After the pickling process, multiple rinsing has to be performed to remove all the acid solutions and stop the pickling process. The water quality requirements, including acceptable chloride content,
increase with the surface requirements.
- Pickling typically involves using an acid mixture containing 8-20
vol% nitric acid (HNO3) and 0.5-5 vol% hydrofluoric acid (HF).
- Chloride containing agents such as hydrochloric acid (HCl) should be avoided since there is an obvious risk of pitting corrosion.
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Alkaline detergent cleaning performed either by immersion or as an electro-cleaning process. The Immersion cleaning is used usually with hot (60-85°C) solutions. For ferrous metals and difficult cleaning operations stronger cleaners, containing sodium hydroxide (silicates or phosphates), soaps and wetting agents are used, in concentrations of 10-40 g of each component per liter of solution.
Non-ferrous metals and especially aluminium are cleaned with inhibited alkaline cleaning solutions, sodium silicates, phosphates, carbonates with soap or synthetic organic detergents, at a concentration of 15-45 g/l.
Vapour degreasing is much more effective than liquid solvent Cleaning. The solvent is heated to boiling, the parts to be cleaned are hung in the chamber in the hot vapor, which condenses on the metal surfaces, dissolves the oil and grease, and flaws back to the solvent containers.
Electropolish
Electro-polishing is a process of smoothing, polishing, and deburring metal surfaces in an electrolytic bath. This process selectively removes high points on the metal surface while producing a uniform high luster on the metal surface. Electro-polishing system removes metal from the piece and adds metal ions to the solution.
In a liquid media, a metal part is immersed and then subjected to a direct current. The metal part is made anodic (+) and a metal, usually copper, is made cathodic (-).
The direct current flows from the anode to the cathode removing metal ions at a controlled rate. The amount of metal removed is dependent upon the specific bath, the temperature, speed, and the particular alloy being electro-polished.
Advantages of the Electro-polish
1- Extends operating times and reduces wear and tear on parts
2- Reduces adhesion and contamination on the surfaces of process equipment (Electro-polished surfaces perform as well as glass surfaces in many applications)
3- Promotes easy cleaning and reduces associated downtime
4- Significantly reduces a metal's tendency to corrode
5- Reduces friction between moving metal parts
6- Improves heat transfer efficiency in heat exchangers
7- Enhances flow characteristics of piping and tubing
8- Eliminates burrs from machined parts
9- Reduces surface stresses in formed metal parts
10- Lengthens metal life against sour gas
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Rinsing of the electro-polish
Viscosity of the acid used in the electro- polish is very high and water rinse is not sufficient. Rinsing is a preparation of a work part for a subsequent operation by a dilution of the chemical substances adhered to the part in the previous operation.
An improperly rinsed part may produce stains on the surface and cause an insufficient adhesion strength of the subsequent coating. In addition, poor rinsing results in a contamination of the subsequent tanks Dilution factor: is the ratio of the concentration of the main chemical in the process tank to the rise of its concentation in the rinse tank as a result of the rinsing step.
There is a need to perform post treatment with nitric acid to remove the residue of the electro polish Post treatment: To remove chemical residues or byproducts of electropolish and to assist drying. Nitric acid post-treatment for electro-polish is to dissolve the film of chemical by-product coming from the electrochemical reactions. By-product mainly consist of heavy metals, there are difficult to remove by water rinsing alone. It is crucial to remove the residue to prevent corrosion. Hot water rinse and centrifugal dryers, heated air chambers and other types of drying stations may be need to force rapid evaporation of residual moisture and prevent staining of the work.
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Passivation
To insure having the maximum corrosion resistance, that austenitic stainless steel has to offer, the stainless steel surface must be in a passive state. Passivation occurs whenever the chromium contained in the stainless steel comes in contact with oxygen from ambient air. The chemical reaction will form a passive oxide chromium layer. The oxide chromium layer will protect the metal surface. In order to maximize thickness and uniformity of the oxide Chemical passivation is a two -step process. - The first step is to remove any free iron or iron compound that is on the surface, otherwise this iron will create a localized site where corrosion can continue. Acid is used to dissolve away the iron and its compounds. The surface itself is not affected by this process. - The second step is to use an oxidizer to force the conversion of chromium metal on the surface to the oxide form. This will create the uniform chromium oxide protective layer.
