cleaning, pretreatment & surface preparation

ELECTROCLEANING

BY NABIL ZAKI

SURTEC INTERNATIONAL, GMBH, ZWINGENBERG, GERMANY; WWW.SURTEC.COM

Electrocleaning is a cleaning process used in metal surface preparation, usuallyin preplate cycles. It is essentially characterized by the use of DC current anda specially formulated electrolyte. The work being processed may be used asanodes or cathodes, or both, depending on the application and basis materialbeing processed. Although electrocleaning is a different and distinct method ofsurface cleaning, it should be considered in the context of the complete surfacepreparation or preplate cycle. A general cycle might include: (1) soak clean, (2)rinse, (3) electroclean, (4) rinse, (5) acid activate, (6) rinse, (7) repeat steps 3 to 6(optional), and (8) plate.Electrocleaners as described herein are alkaline and will generally followalkaline soak cleaners and precede acid activation in the preplate cycle. The basicfunction of electrocleaners is to remove soils from the surface that could notbe removed by simple immersion soak or degreasing steps. Examples of suchsoils are as follows:

• Adherent residues not removed in the preceding soak cleaner. Such residuesinclude oil, fingerprints, drawing compounds, and soils driven into surface

porosity or applied under pressure. These soils are generally not removed byconventional emulsification, wetting, and displacement soak cleaners.

• Finely divided particles, such as polishing compound abrasives, metallic finesfrom grinding or metalworking operations, carbon, and other alloying elements,

may also be found on the surface. Often this fine particulate matter, generallyreferred to as smut, may be held to the surface by simple mechanical forces,

electrostatically, or in a thin oil or grease matrix.

• Metal oxidation products, the result of exposure to the atmosphere, or a thermalprocess such as heat treatment, forging, welding, etc.

As explained earlier, electrocleaning must be viewed as part of the overallsurface preparation process. Although an electrocleaner step may not totallyremove a particular type of soil, it conditions or modifies that soil for easierremoval in the subsequent steps in the cycle. For instance, an adherent oil residuemay be loosened enough to be lifted in the following rinse tank. Surfaceoxides may be reduced or oxidized to a more soluble form to be easily dissolvedin the subsequent acid tank.

ELECTROLYSIS OF ELECTROCLEANERS

As current is applied to an electrocleaner, the following electrochemical reactionstake place, essentially electrolyzing the water component of the electrolyte. Thealkalies serve as the conductive medium.

At the anode:

4OH

–

¨ 2H

2

O + O

2

+ 4e

-

64

At the cathode:

4H

2

O + 4e

–

¨ 4OH

–

+ 2H

2

During electrolysis, twice as much hydrogen is liberated at the cathode than isoxygen at the anode.

TYPES OF ELECTROCLEANERS

Electrocleaners are classified on the basis of two main criteria: (1) polarity of thework in the tank; and (2) the type of substrate being treated.

There are three types of electrocleaning modes as defined by polarity of thework in process and the applicability of each to a givensubstrate.

Anodic Electrocleaning

The work is connected to the anode side of the rectifier and is positively charged.The process is also known as reverse electrocleaning, since the polarity is oppositethat of plating. As described under electrolysis, oxygen is liberated at the surface ofthe work (the anode) when current is applied. As the gas rises to the top, it createsa mechanical scrubbing action that loosens and lifts the soils.Two other phenomena also take place. As oxygen bubbles are formed on thesurface, they coalesce and grow before they rise in continuous layers. It is believed

that the static charge holding fine particles to the surface is released through thelayer of bubbles, facilitating their removal through the scrubbing action.Chemical effects, oxidation, and drop in pH also take place at the anode surface.If excessive, the effect of oxidation can be seen, for instance, on brass, zinc, andsilver as they discolor, stain, or etch. Special inhibited anodic electrocleaners are

available for brass and zinc.When nickel is anodically electrocleaned it will quickly become passive and preventfurther plating unless reactivated. A similar effect is experienced with stainlesssteels. Regular steels are not adversely affected by the process, whereas high-carbonsteels are more sensitive and require moderation in electrocleaning. Alloys of lead,nickel–silver, and silver plate are attacked or tarnished by anodic electrocleaning.As oxygen is liberated at the anode, the net pH value tends to decrease at theinterface. This effect can be noticed on steel if an electrocleaner’s alkalinity is toolow by design or for lack of bath maintenance. The steel is more rapidly oxidized,and precipitated iron hydroxide forms on the surface. Parts exiting the tank willhave a rusty or etched appearance, especially in high current density areas. Thesituation can be readily rectified by increasing the alkalinity of the bath or byreducing the current density below normal operating levels until the bath chemistryis adjusted.

Cathodic Electrocleaning

The work is connected to the cathode side of the rectifier and is negativelycharged. This is also known as direct electrocleaning. In this case, hydrogen isliberated at the cathode. As seen from the net amount of electrolysis, twice asmuch hydrogen than oxygen is generated at the cathode. Consequently, morescrubbing action and cleaning ability are expected. The use of cathodic electrocleaning,

however, has not found a widespread use in the industry as the mainelectrocleaning mode for two reasons: (1) the concern with hydrogen embrittlementas a result of copious hydrogen release at the surface, and (2) the risk ofplate out of charged impurities from the solutions on the cathodic surface. Thelatter may not be noticeable to the casual observer as the parts exit the tank, butit leads to poor adhesion on plating. Contaminants leading to such adhesionfailures are metallic fines, certain types of surfactants, colloids, metallic soaps, andhexavalent chromium dragged into the cleaner.

Cathodic cleaners, when kept clean and well maintained, are used for processingbuffed brass, zinc, and white metal without tarnishing, and for electrocleaningnickel and high nickel steels without risking passivation. When cathodicelectrocleaners are used on steel and copper to take advantage of their superiorscrub-clean action, a secondary anodic electrocleaner should follow even for a fewseconds. This step will deplate any impurities that may have plated on the workby cathodic action.

Periodic Reverse Electrocleaning

This method of electrocleaning of ferrous metals uses a combination of bothanodic and cathodic cleaning modes. A periodic reverse (PR) unit is installed onthe rectifier’s output. The PR unit has a switching mechanism that reverses thepolarity at controlled and timed intervals. The work in the tank assumes alternatinganodic and cathodic polarities for the specified cleaning time.Typical settings include reciprocating 10-second cathodic then 10-secondanodic for the duration of the cleaning time. By alternating polarity, morecathodic or anodic may be used to effect maximum cleaning. The unit can beprogrammed so that the last leg of the cycle is anodic before the timer shuts offthe rectifier. This ensures deplating of any charged particles that may have platedon the work during cathodic cleaning.The continuous oxidation and reduction at the surface converts the oxidesand scales on parts to more soluble forms that are picked up by complexersor chelating agents built into the cleaner formulation. PR cleaners eventuallybecome saturated with dissolved iron oxides and must be replaced. Where

practical, they can be regenerated by plating out the iron cathodically. PRcleaning is very efficient in descaling and derusting high-strength and springsteels without the use of acids in the cycle, thereby eliminating or minimizinghydrogen embrittlement.

OPERATING PARAMETERS AND PROCESS CONSIDERATIONS

Electrolysis is the main driving process in electrocleaners. The amount of gassingresponsible for the scrubbing action at the electrodes is a function of the amountof current passing through the cell. Therefore, parameters controlling currentshould be considered.

Solution Conductivity. This in turn is a function of cleaner concentration andtemperature at a given voltage. The higher the concentration and temperature(up to a practical level), the higher the conductivity and the amount of gassing. Voltage Applied. Current increases with voltage, although the latter is kept tomaximum values of 10 to 12 V. Higher values are known to cause “burning” orroughness on parts.

Surface Area Being Cleaned. This parameter controls current density and, for agiven rectifier setting, will directly affect the cleaning efficiency.Adequate recommended current density ranges for different base metals aresummarized in Table I. Values below these produce light to marginal electrocleaning.Higher values generally lead to etching and roughness of the surface.Anode to cathode area ratios of 1:1 are adequate for most applications.The ratio is not critical as long as the specified current density values aremaintained.

PROCESS CONSIDERATIONS

There are general considerations in the selection and proper use of electrocleaners.

Electrocleaner Formulation

Several proprietary formulations are available covering a wide range of applications.These formulations should provide the following properties:

• A suitable degree of alkalinity for the metal processed, e.g., high alkalinityfor steel, lower for zinc and brass.

• A proper ratio of hydroxide to silicate to prevent insoluble silicatefilms from adhering to the work and affecting plate adhesion. Silicates

are often used to prevent burning of steel at high current density.Nonsilicated cleaners, using different types of inhibitors, are also available.

Water softeners and conditioners should be considered in hardwater areas.

• An adequate amount of wetting agent and emulsifiers. Although highlevels serve as cleaning agent for excess oil and grease, they inhibit the gassingaction at the electrode surface and reduce desmutting characteristics.Efficient desmutting electrocleaners will have just enough surfactantsto reduce solution surface tension and promote a thin foam blanket tohold down gas misting during electrolysis. Bulk oil removal should be areserved function of the preceding soak cleaner.Typical operating conditions of electrocleaners are given in Table II. Suppliersof proprietary electrocleaners usually tailor the parameters to specific applications,which may vary from the values shown in Table II. Alkalinity figures expressed asNaOH may represent 20–80% of the total product formulation.

Process Control

Control of electrocleaners is usually done by titration of the alkali contents.Maintenance additions will replenish alkalies, as well as surfactants and othercomponents included in the formulation.Table II. Typical Operating Conditions for Electrocleaning

Although the essential components can be maintained, contaminants buildup and eventually interfere with the proper performance of the bath. Oils andgrease, if not adequately removed in the preceding soak cleaner, may result inwater breaks out of the electrocleaner tank. Grease etch is a result of such abuildup. It shows as jagged etch spots after plating. It is due to uneven currentdistribution around non-wetted spots on the surface being electrocleaned.Hexavalent chromium trapped in cracked rack coatings and dragged into the

electrocleaner is another source of contamination. This leads to drasticallyreduced cleaning and haze under nickel plate.Stripping chromium-plated parts for rework in the process electrocleaner has asimilar effect. Hexavalent chromium contamination can be readily detected as the

cleaner turns yellow–orange and foaming seems to subside. Compatible chromiumreducers are used to counter this effect. They reduce the chromium to trivalent if thecleaner is not heavily chelated and allow most of it to precipitate as the hydroxide.The solution color changes to light green, indicating the reduction process hastaken place.

Equipment Maintenance and Operation

Corrugated or mesh steel can be used as anodes or cathodes to provide optimumsurface area and solution circulation. Periodic cleaning of the anode/cathode isnecessary to remove plated-on smut, oxides, and other charged particles. Usingthe tank as the anode or cathode is not recommended, as it leads to uneven currentdistribution and a source for stray current. Many electrocleaning problems,such as under- and over-cleaning, have been traced back to such a practice. Asummary of common problems is given in Table III.Polypropylene or lined tanks are recommended for alkaline electrocleanersfitted with steel, stainless steel, or Teflon heaters. Recirculating pumps are recommendedto prevent stratification and ensure overall homogeneity. It should benoted that solution inlet and outlet must be located at two opposite diagonal topand bottom corners of the tank for efficient solution movement.

Table III. Common Electrocleaning Problems

Cleaner filtration is gaining in popularity with the aim of prolonging thebath life between discarding and bath replacement intervals. Several filtrationtechniques have been proposed, ranging from simple bag filtration to completesystems of oil skimmers, coalescers, and ultrafiltration. Since the cost of suchsystems varies appreciably, a feasibility study must be undertaken before adoptinga particular system. In general, however, it has been reported that any type offiltration does increase the bath life at least by 20% and up to five times or more.

LIQUID CLEANERS

The use of liquid cleaners to replace powder versions has gained momentum andwide acceptance in the industry. These new cleaners are formulated to economicallyprovide all the performance criteria of the powders. The advantages of liquidcleaners include the capability of automatic feeding tied to conductivity controllers.The automated system continually monitors the solution strength and makes

additions on demand. Consequently, better bath control is achieved, eliminatingwide swings in concentrations. Automatic recording capabilities of concentrationand temperature can be achieved for statistical process control.Tank additions of liquid concentrate eliminate the hazards associated withadditions of alkaline powders to hot cleaner solutions. As a result of bettercontrols, these liquid systems have substantially increased bath life in manyinstallations.Another advantage confirmed by users of liquid cleaners is sludge reduction

on waste treatment by 70–80%, which adds to the economical advantage ofthese systems.

ELECTROCLEANING OF STRIP AND WIRE COILS

Strip and wire are continuously fed through the line. There is no direct contactwith anodes or cathodes in the electrocleaner tank. A bipolar electrical effect isused to provide the polarity as follows. Two separate steel grids are positioned severalfeet apart in the direction of work flow. The first grid is anodically connected,the second, cathodically. The strip travels between two closely spaced jaws of each

grid without touching them. As a result of the bipolar effect, the strip assumesthe opposite charge of the grid. It becomes cathodic through the first grid, then anodic through the second, achieving the desired electrocleaning effect.