Electrolytic Cleaning Equipment Information
Electrolytic cleaning equipment uses electrolytes and applied current to electrochemically clean welds and other metal surfaces. Electrolytic cleaners are also known as electrocleaners, weld cleaners, electrolytic pickling lines, and electrochemical cleaning systems. The electrolytic process can also passivate and smooth (electropolish) and mark surfaces. Electrocleaning is highly effective in removing thermal oxidation discoloration or heat tint from welding or heat treating, oxide scales on castings or forgings, hard-water mineral scale, rust, tarnish, oil, wax, grease, machining fluids, carbon deposits, drawing compounds, extrusion lubricants, grinding coolants, polishing or buffing compounds, coatings, paints, fingerprints, metallic fine or swarf, smuts, and other soils.
Video credit: SURFOX 304—MIG & TIG
Electrolytic cleaning systems utilize an electrolytic cell. Electrolytic cells are comprised of two electrodes, the anode (positive terminal) and the cathode (negative terminal and an electricity-conducting electrolyte solution). Negatively charged ions and colloidal particles travel through the electrolyte and are attracted to the positive anode. Oxidation and release of oxygen occurs at the anode (4OH- → 2H2O + O2 + 4e-). Positively charged metallic ions travel through the electrolyte and are attracted to the negative cathode. Reduction and hydrogen evolution occurs at the cathode (4 H2O + 4e- → 4OH- + 2H2). Oxides and corrosion films (sulfides, chlorides, etc.) on the part should be broken down at the part’s surface resulting in a cleaning action. The bubbling action from the release of oxygen and hydrogen also provides a degree of cleaning action.
The electrolyte cleaning is typically based on phosphoric acid or alkaline solution with additives to increase the conductivity, wetting aids, deflocculating agents, silicates (act as corrosion inhibitor, emulsifier, oil peptizer), water softeners or conditioners (phosphates, gluconates, etc.), chelating or sequestering agents to capture dissolved metal ions (e.g., alkyl sulfonate or alkyl ether alcohol) and sometimes thickeners to increase viscosity. Phosphates and silicates are also common additions in phosphated and silicated electrocleaner electrolyte products. Some chemical manufacturers offer silicate and phosphate free versions of their electrocleaners. Steel, nickel, and copper alloys are electrocleaned with alkaline cleaners. Brass, bronze, zinc and tin, and aluminum alloys are best cleaned using inhibited alkaline cleaners. Alkaline solutions emulsify oils, grease, and other lipids while the electrocleaning action removes thermal discoloration from oxidation, welding oxide scale, and iron fines.
The electrocleaner chemical selected should have enough alkalinity to dissolve insoluble silicates and clean the metal being processed. For instance, steels require high alkalinity electrolytes compared to zinc and brass. The electrolyte solution can be applied to the part by immersion, spray, or applicator. Applicators typically consist of a foam (sponge) over a metal conductor or a conductive carbon fiber brush. Ultrasonic agitation can be applied to enhance cleaning action in electrolytic soak cleaners with immersion tanks.
A direct current power source or rectifier provides the electrical current to anodic or cathodic electrolytic cells. In some electrolytic cleaning systems in the periodic-reverse cleaning mode, an alternating current (AC) power supply or a rectifier applies an electric current to the system. With AC current or periodic cleaning, the electrodes switch back and forth as anodes and cathodes. Periodic-reverse may operate 50% of the time as anodic and 50% as cathodic. Some electrolytic cleaning machines allow continuous electrolytic cleaning lines for wire, metal sheet, strip, rod, or other alloy roll goods used in a series of electrolytic immersion baths or chambers where DC polarity is alternated from positive to negative between chambers. Smaller hand-held electrocleaners or electrolytic weld cleaners mainly use AC current for cleaning. Spray electrolytic cleaning systems are also used on continuous sheet or sheet materials, which may provide longer electrode life, higher current densities, and lower power consumption compared to immersion chamber systems. Electrolytic cleaning lines often incorporate a secondary cleaning process such as rotary brushing or ultrasonic cleaning. An electrolytic immersion chamber system can easily incorporate ultrasonic cleaning transducers within the chambers, which is an advantage over electrolytic spray cleaning lines.
Several distinct types of electrocleaning processes are available such as cathodic cleaning, anodic cleaning, periodic-reverse, and interrupted cleaning.
|Anodic/Reverse-Current Cleaning||Workpiece is anode; oxygen bubbles released at surface; more widely applied than cathodic cleaning; dissolve workpiece removing any metallic films and pushes debris away from workpiece; Preferred process over cathodic in many applications; Chemical changes, oxidation, and drop in pH also take place at the anode surface, which can discolor, stain, or etch brass, zinc, and silver if not controlled.||Alkaline or basic solution or inhibited alkaline solutions.||Copper, zinc, brass, high carbon steel, and ferrous alloys?if hardened steel (> 40 Rockwell C), post hydrogen embrittlement relief heat treat or hydrogen bake at 400° F required. Nickel surfaces rapidly passivated during anodic cleaning; copper requires short anodic cleaning time to avoid tarnish formation.|
|Cathodic/Direct Current Cleaning||Workpiece is cathode; hydrogen bubbles released at surface; metal debris and reduction can cause deposits or plating out of undesirable metal films; deposits within the system can cause frequent replacement or remanufacturing of the electrocleaner.||Acidic solution.||Magnesium (best process for), stainless, low carbon steel, aluminum, chromium, zinc, tin, brass, nickel (avoids passivation), cobalt, copper, chrome plate (chromium), lead (avoids staining), silver, and some other precious metals and any alkaline-soluble metals.|
|Periodic-Reverse (PR) Cleaning||Workpiece alternates between anode and cathode?typically with a final anodic step; oxides removed without over etching from trapped acid in crevices as occurs in pickling or cathodic processes. Higher applied voltages of 6 to 15 V are used compared to anodic cleaning. Continuous reduction and oxidation at the part surface converts scale or oxides in soluble form captured by chelating agents. Metal must be plated out or electrocleaner solution replaced.||Alkaline or basic solution often with sequestering additives or chelating agents to enhance smut and scale removal.||PR cleaning efficiently removes scales and derust hardened high strength and spring steels without descaling acids, which limits hydrogen embrittlement.|
|Interrupted-Current (IR) Cleaning||Electrocleaning reactions change or deplete the solution chemistry at the surface. The IR cleaning process allows replenishment of cleaner chemical solution at the part's surface by turning power off for 1 to 2 seconds every 8 to 9 seconds.||Alkaline or basic solution.||Can be applied to a variety of metals.|
Heat is generated during the electrolytic cleaning process due to joule heating from the current and resistance in the system. Electrolyte conductivity increases with temperature until boiling occurs because the bubbles are non-conductive and reduce conductivity. Heating and cooling temperature control is required in larger electrolytic cleaning to keep the solution from boiling and to maintain the electrolyte above 40° C (104° F) and below 100° C (212° F). Some sources recommend operating between 40° C (104° F) and 60° C (140° F). However, the chemical manufacturer’s recommended operating temperatures vary for each electrolyte cleaning product. For example, Hubbard-Hall’s recommends operating temperatures of 60° C to 91° C (140° F to 205° F) for their Cleaner 200 PWA product. The electrolyte bath chemistry must be monitored for pH, contamination (chlorides, sulfides, metal ions build-up?especially chromium), and the electrolyte chemistry adjusted with additives to within allowable limits. Eventually, additives may not be effective and the electrolyte will require replacement. Current densities and electrode potentials are additional parameters to be monitored in large scale electrolytic cleaning systems.
The optimum operating temperatures, current densities, and other operating parameters vary with the electrolyte, electrolytic process type, and metal substrate being cleaned. The following table provides some typical operating conditions for electrocleaning processes.
|Electrocleaning Parameter||Steel, Low Carbon||Steel, High Strength||Stainless Steel||Copper||Brass||Zinc||Ni and Cr Alloys|
|Current Density (amps/ft2) Anodic Cleaning||47-93||28 - 47||—||47-74||19-37||19-37||—|
|Current Density (amps/ft2) Cathodic Cleaning||47-93||—||47-74||47-74||19-37||19-37||19-28|
The tank, vat, or chamber materials must be made of corrosion resistant material such as polypropylene. Electrode materials and clamps must be conductive and corrosion resistant. The grounding clamps submerged in the electrolyte should not release any ions or stain the workpieces.
Electropolishers smooth or polish metal surfaces by removing microscopic high point and through an electrochemical process with an applied DC current. The electropolishing process preferentially dissolves the high points or peaks on a part’s surface resulting in a smoother surface. The workpiece is the anode or positive electrode in the electrical circuit, so oxidation, conversion, or dissolution of metals to ions and oxygen bubble formation occurs. The positive metal ions move through the electrolyte and are “plated out” or deposited onto the cathode. Hydrogen bubbles and reduction of ions to metal occurs at the cathode.
Electropolishing machines utilize an electrolyte containing sulphuric acid, perchloric acid, and/or phosphoric acid often in an ionizing solution with viscosity modifiers or thickeners. Alkaline electrolytes are used on certain metals. The specific composition will vary with the type of metal or alloy being electropolished.
Electropolishers also provide a deburring action because the burrs are high points and are preferentially dissolved.
Passivation equipment utilizes passivation chemicals or electrochemical processes to passivate or to reduce surface reactivity to protect a metal against corrosion. Excess iron and oxide scale from welding and other thermal processing can result in reduced corrosion resistance or non-passivated condition. Depositing an inert coating or growing a coherent oxide surface layer can also passivate a surface. Electropolishing can also be used to passivate a metal or alloy surface by forming a coherent protective oxide film.
Electrolytic processes can also mark or etch a surface using a stencil or mask with the desired pattern or lettering placed over the workpiece surface before electrolysis. Some suppliers sell optional marking and etching kits and stencils to facilitate part identification. Marking kits are often sold by manufacturers of electrolytic weld cleaners.
Electrolytic weld cleaners are portable electrolytic cleaning machines with a hand-held brush or applicator for selective cleaning and removing heat tint from a finished weld. Welder cleaners are faster and more thorough compared to alternative methods such as applying pickling pastes or abrasive cleaning. Electrolytic weld cleaners provide additional benefits such as electrochemical surface passivation and electropolishing, which are not possible with alternative methods. Weld cleaning, weld polishing, and weld passivation are important steps in post weld treatment to make sure the finished weld will not be attacked by a corrosive environment and prematurely fail. This is especially important in welding stainless steels. Stainless steels are normally protected by a thin, coherent chromium oxide layer. The welding process results in a surface or regions depleted in chromium content, which make these regions more susceptible to corrosive attack. Electropolishing, or electrocleaning, increases the surface chromium, which promotes the formation of protective film. Weld cleaners are useful in cleaning TIG welds, MIG welds, spot welds, and resistance welds.
Electrolytic cleaning, polishing, and marking processes have advantages and disadvantages compared to competitive technologies like abrasive finishing, machining, pickling, aqueous washing, and solvent cleaning.
- Highly effective cleaning process on a wide range of soils and surface contaminants such as thermal oxidation from welding or heat treating, oxide scales on castings or forgings, hard-water mineral scale, rust, tarnish, oil, wax, grease, machining fluids, grinding coolants, coatings, and paints.
- Very thorough cleaning with minimal damage to surfaces in controlled electrocleaning operations using the properly selected electrolytic cleaning equipment and electrolytes from high-quality OEMs.
- Electrolyte cleaning solutions are more benign (no VOCs), easier to handle, and more environmentally friendly compared to aggressive pickling pastes, etching solutions, and chemical solvent cleaners.
- Electrolytic cleaning action is faster compared to pickling or other purely chemical processes?cleaning times range from 0.5 to 10 minutes with 1 to 3 minutes being typical. Faster cleaning times are possible at very high current densities and aggressive electrocleaners.
- Many electrocleaners and electrolytic cleaning units have additional functionality such as:
- Electropolishing the surface after cleaning.
- Passivating a metal surface by removing tramp metal or iron and providing a coherent, protective oxide film.
- Marking or etching of metal surfaces using a stencil or mask.
- Compared to abrasives or machining:
- No danger of imparting grinding burn or from excessive heat during abrasion.
- No danger of imparting tensile residual stress into a part’s surface as may happen with grinding and machining.
- Specialized equipment and electrolytes required?high-quality electrolytic cleaner or electrocleaner.
- Each specific alloy type requires proper selection of electrolytic cleaning equipment, process parameters, and electrolytes, which require knowledge of the electrocleaning process to avoid burning or roughening of the part’s surface.
- Alloy ID tester might be required to select the correct electrolytes and parameters for the alloy.
- Consumables required:
- Electrolyte chemicals and additives?different chemicals required for specific metals and processes (Cleaning and marking versus polishing).
- Deionized or purified water source.
- Neutralizing solutions required to neutralize acidic residues.
- Applicators or brushes.
- Clamps, electrodes, sink rolls, etc.
- Larger production systems or electrolytic cleaning lines:
- Require monitors (pH, temperature) and automation for part, metal stock, or web handling.
- Have significant electrical power consumption requirements compared to conventional cleaning methods
- Hydrogen embrittlement can occur in cathodic cleaning since hydrogen is generated at the cathode when electrocleaning heat treated hardened steels. Hydrogen diffuses into the steel. A post cleaning heat treatment can be applied to remove the hydrogen, but weakened material and parts can result if this step is skipped. Alternative processes might be advised for hardened steel.
- Hydrogen, oxygen, and other vapors evolved may require fume extractor and venting.
- Metal film formation during certain cathodic cleaning processes can occur.
- De-zincification or over etching of zinc components.
Stainless steel, carbon steels, alloy steels, titanium, nickel alloys (Hastelloy®, Inconel®), cobalt, copper, coper alloys (brass, bronze, beryllium copper), gold, nickel silver, and copper-nickel (Monel®) alloys have been electropolished, electrocleaned, and electrodeburred.
Electrolytic cleaning and polishing methods are important in a variety of industry applications such as:
- Welds and welded joints--restores passivity and improves appearance
- Electronic components (connectors, contacts, etc.)
- Mechanical components (bearing parts, fasteners, hardware, pins, etc.) cleaning, deburring, and polishing
- Aerospace component cleaning
- Automotive component cleaning
- Printed circuit board cleaning
- Pre-plating cleaning to improve metal plating quality
- Pre-coating cleaning to improve e-coating (electrophoretic) or painting quality
- Manufacturing of medical devices and surgical tools
- Dental lab and office--cleaning and polishing dental fixtures, orthodontics, crowns, etc.
- Food and beverage or sanitary fluid handling components (stainless steel valves, fittings, pump parts, etc.)
- Jewelry and decorative parts
- Pre-treatment before refinishing or recoating?facilitates the removal of coatings, paints, and other finishes
- ASTM A380—Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems
- ASTM B117—Practice for Operating Salt Spray (Fog) Apparatus
- ASTM B254—Practice for Preparation of and Electroplating on Stainless Steel
- ASTM-B322—99(2014)--Standard Guide for Cleaning Metals Prior to Electroplating
- ASTM A967/A967M–13—Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts
- ASTM B912—Standard Specification for Passivation of Stainless Steels Using Electropolishing
- ASTM F86—Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants
- ISO 15730:2000—Metallic and other inorganic coatings?Electropolishing as a means of smoothing and passivating stainless steel
- SAE AMS1547—Cleaner, Anodic, Electrolytic, Alkaline, Steel, Tank Type
- AWS D1.6/D1.6M:2007—(Section 5.0 Weld Cleaning)?Structural Welding Code—Stainless Steel
- AWS D18.3/D18.3M:2005—Specification for Welding of Tanks, Vessels, and Other Equipment in Sanitary (Hygienic) Applications
- AWS D18.2:2009—Guide To Weld Discoloration Levels on Inside on Austenitic Stainless Steel Tube
- DIN 65473 (1991)—Aerospace: Electrolytic Degreasing and Cleaning
- NACE 06428—Passivation of Stainless Steels Measured with Electrochemical Noise