IN THIS FILE:
As stated in previous section, the preliminary evaluation of an iron artifact will determine the methods by which the artifact is cleaned. Only after an artifact has been evaluated and cleaned can the appropriate conservation measures be taken. The various treatments available for the conservation of iron can be organized into five general categories:
1. Electrochemical cleaninga. Galvanic cleaning
b. Electrolytic reduction
2. Alkaline sulfite treatment
3. Chemical cleaning
5. Water diffusion in alkaline solutions
The corrosion of metal, as explained earlier, is an electrochemical reaction. Electrochemical and electrolytic reduction cleaning processes, therefore, are the most common techniques utilized to halt, stabilize, and even reverse the oxidation of the metal. In the field of conservation it has been the general practice to distinguish between electrochemical and electrolytic cleaning. An electrochemical reaction is based upon the association of two metals occupying different positions on the galvanic or electromotive series of the metals without an externally applied electromotive force (EMF); therefore this cleaning process is also known as galvanic cleaning. Electrolytic reduction, also called electrolysis, is an electrochemical reaction maintained by an externally applied EMF or electric current. Basic information on these two processes can be found in Plenderleith (1956), Plenderleith and Torraca (1968) and Plenderleith and Werner (1971). Hamilton (1976:30-49) and North (1987:223-227) provide detailed discussions of electrolytic reduction.
Galvanic cleaning is often mentioned as an effective method for conserving shipwreck material. In two frequently cited works on underwater archaeology, it is the only technique recommended (see Peterson 1969:83-84; Marx 1971:125). For marine-recovered iron artifacts which are severely chloride-contaminated, however, galvanic cleaning is not a favorable alternative. It is often not desirable even for metals from terrestrial sites, where chlorides present no real problems.
To be effective, galvanic cleaning requires that a substantial metal core be present in the object being treated. In most circumstances, this process can be recommended only if a few small objects are to be cleaned and if equipment for electrolytic cleaning is not available. Except for limited use, galvanic cleaning is best considered as an obsolete technique. Since it is used in some laboratories, however, it is briefly discussed here. Some of its advantages and its major disadvantages are presented.
Galvanic cleaning involves placing the iron object in a vat and surrounding it with a more active anodic metal, such as zinc or aluminum, and filling the vat with an electrolyte. In this process, nascent hydrogen acts as the reducing agent as it evolves from the surface of the iron. The chlorides are removed, and reduced metal is left by the reaction. In order for galvanic cleaning to be effective, it is necessary to control the electrode potential of the artifact being treated. In galvanic cleaning this is determined by the couple between the anodic metal and the iron and by the electrolyte; once the electrode potential of the artifact has been established, however, it cannot be manipulated.
The simplest method of galvanic cleaning consists of wrapping the object loosely in aluminum foil and placing it in a glass beaker of 10 percent sodium hydroxide, or caustic soda (NaOH), or a 10 to 20 percent solution of sodium carbonate, or soda ash ( Na2CO3). (Noël Hume [1969:283] suggests using an electrolyte of baking soda (sodium bicarbonate, NaHCO3), but tests at the Texas A&M Conservation Research Laboratory have shown that baking soda electrolytes are not effective in galvanic cleaning.) The object is left in the caustic solution until the aluminum foil completely oxidizes. The reaction can be speeded up by heating the solution. The object is then rinsed, and the process is repeated until satisfactory results are achieved. Only small, lightly corroded specimens can be treated in this fashion.
The most commonly used method of galvanic cleaning differs from the method above only in that granulated zinc or aluminum granules are used to cover the object and a 10 to 20 percent solution of sodium hydroxide is employed (Plenderleith and Torraca 1968:241; Plenderleith and Werner 1971:194-197). The solution is heated to boiling in a metal pan or heat-resistant glassware, and the level of the solution is maintained with the addition of distilled water. The cleaning continues until the electrolyte is exhausted or the activity of the zinc abates. The process is repeated with fresh or cleaned zinc and fresh caustic soda until all of the materials are reduced and all traces of chlorides have been eliminated. If high levels of chlorides are allowed to remain in the artifact, future outbreaks of corrosion are inevitable.
This process requires facilities to heat the container and an adequate ventilation system, such as a fume hood, to exhaust the caustic vapors. In the reduction process, the caustic solution is exhausted and has to be periodically discarded, and considerable zinc is lost through oxidation. The activity of the remaining zinc is significantly reduced by an accumulated film of oxychloride and carbonate. To be renewed, the zinc must be cleaned with dilute hydrochloric acid solutions, rinsed in distilled water, and then dried (Plenderleith and Werner 1971:196). Alternatively, the zinc can be melted in a ladle under a reducing flame and re-granulated by slowly pouring the molten zinc into water (Organ 1973:193); each time the zinc is reclaimed, slag formation will further reduce the quantity of zinc.
For iron artifacts, or any other metal with extensive chloride corrosion compounds, the galvanic cleaning process is impractical. The artifact is always obscured, so it is not possible for the conservator to observe its progress. Even under ideal conditions, the process requires constant supervision and is messy. The fumes produced are unpleasant and irritating to the skin, eyes, and throat. Furthermore, it is often difficult to provide simultaneously adequate ventilation and adequate heat. Large artifacts, especially those which are chloride-contaminated, require a long processing time and a prohibitive amount of zinc, which has to be continuously cleaned and replenished.
Galvanic cleaning of most metal artifacts should be considered only if equipment for electrolytic reduction is unavailable, and even then, it may be a waste of time for a majority of iron artifacts, especially if they are very large. Noël Hume's comment (1969:276) bears repeating: "Those amateurs who have been told that it is a simple method that can readily be performed on the kitchen stove are advised to forget it."
ELECTROLYTIC REDUCTION CLEANING
Electrolytic reduction cleaning is one of the most efficient and effective methods of conserving metal artifacts. Electrolytic units are very cost-effective and simple to set up and maintain. The cleaning process can be selected exclusively for its mechanical cleaning action of the evolved hydrogen, for the metal reduction process or a combination of the two. Efficient electrolytic reduction, however, involves more than simply wiring up artifacts for electrolysis. A knowledge of corrosion processes and electrochemical thermodynamics is essential. The conservator must be familiar with electrode potentials and pH and know how these variables relate to electrode corrosion, passivation, and immunity. These factors are particularly crucial when dealing with chloride-contaminated metals. This is not to say that satisfactory results cannot be obtained by the novice, but rather that knowledge and experience enables the conservator to understand and better control what is going on in the electrolytic cell and to correct adverse conditions.
The essence of the technique involves setting up an electrolytic cell with the artifact to be cleaned as the cathode. An electrolytic cell consists of a compartment or vat with two electrodes, the anode and the cathode, and contains a suitable electricity-conducting solution called the electrolyte. An electric current from an external direct current (DC) power supply is applied to cause oxidation and reduction. The anode is the positive terminal of the electrolytic cell, to which electrons, negatively charged ions, or colloidal particles, travel when an electric current is passed through the cell. Oxidation occurs at the anode and oxygen is evolved. The cathode is the negative terminal of an electrolytic cell, to which positively charged metallic ions travel. At the cathode, reduction takes place and hydrogen is evolved. In the reduction process, some of the positively charged metal ions in the compounds on the surface of the artifact are reduced to a metallic state in situ. In addition, chlorides and other anions are drawn from the specimen and migrate toward the positively charged anode by electrolytic attraction.
The chief advantage of electrolytic reduction is that the density of the externally applied electromotive force (EMF), or electric current, can be controlled. This control enables the conservator to select a low current density that creates a preselected electrode potential conducive for the consolidation and/or reduction of some mineralized metals. When there is an underlying metal core, it is theoretically possible to reduce enough of the ferrous corrosion compounds back to a metallic state through electrolytic reduction. This will consolidate the corrosion layer while eliminating the chloride components of the compounds. A high current density can be selected so that the evolved hydrogen will mechanically remove any completely oxidized crust.
When using electrolytic reduction cleaning, factors to be considered are the equipment and the experimental variables.
1. EquipmentDC Power Supplies
a. Power supplies2. Experimental variables
b. Terminal wires and clips
c. Anode material
d. Chloride monitoring
a. Types of electrolytic setups
c. Current densities
d. Electrode potentials
Most small DC power supplies have an output current with less than 0.1 percent ripple. The larger power supplies have 0.5 percent or more ripple. For well-controlled reduction, the low ripple power supplies are recommended; a little ripple, however, is not harmful. The choice of a power supply depends upon the desired current density control, the size of the artifact, and the number of artifacts on any one unit. Current controls and an amperage meter provide a means of determining and adjusting the current as the treatment progresses. During electrolysis, the current increases as the metallic species are reduced and the resistance of both the object and the electrolyte decreases. The current resistance (IR) drop in the electrolyte is due to an increase in chloride and other ions. This is the main reason for the current increase; the decrease in the resistance of the object plays only a minor role. Therefore, variable adjustments are necessary if an object is to be electrolytically reduced at a fixed current density or a predetermined electrode potential.
Because of the expense of regulated power supplies it is not surprising that many conservation laboratories build their own power units. Foley (1967) and Organ (1968:291-308) give directions on how to build an inexpensive power supply. Alternatively, battery chargers can be employed. Battery chargers, however, are not designed to run continuously for electrolytic cleaning. If they are to be useed in electrolysis, it is usually necessary to remove timers, relays, and charging rate devices to make them serviceable. Since an electrolytic cell for cleaning artifacts lacks the resistance of a battery, and the battery chargers generally do not have the necessary internal resistance controls to compensate for it, the chargers run well above their maximum safe operating amperage. Additional resistance must be added to the circuits to keep them from overheating. Variable autotransformers, such as Powerstats, on the input alternating current (AC) line and variable rheostats, or line resistors, on the outgoing DC negative terminals serve this purpose well. Anyone with a basic knowledge of electrical circuits can easily alter most direct current battery chargers for electrolytic cleaning of metal artifacts.
Terminal Wires and Clips
For most objects, terminal wires made of U.S. National Electric Code Standard 16 AWG, Separation 2, 300V maximum rating insulated copper wire can be used. This wire is the standard two-ply multistrand wire commonly found on many electrical appliances and is quite flexible and easy to use. One strand of the wire is used for the negative connection, the other for the positive connection; or they can be joined at the terminal ends to make one pole connection if a larger sized wire is required. For larger artifacts that need more current, No. 2 to No. 0 AWG multistrand wire is required. Multistrand copper wire is recommended in all cases because it has a larger current capacity and is more flexible and easier to manipulate than comparably sized solid wire. Before using any wire, one should check the amperage capacity of the wire, or, if in doubt, consult an electrician. A good rule of thumb to follow is that the wire should not heat up during electrolysis; if it does, the wire is not heavy enough.
Steel alligator test and battery clips (Mueller clips) are recommended for attaching the terminal leads to the artifact and the anode. Appropriatly sized clips on the terminals facilitate setting up and taking down artifacts in electrolysis. These clips come in a variety of shapes and sizes. The size of the clip selected is determined by the current to be used, the size of the artifact, and the placement of attachments. Mueller clips Nos. 25, 27, 48 and 85 are the most useful sizes and should be kept in stock. Steel clips, which are usually are cadmium- or zinc-plated, should be stripped of this plating by a quick bath in a dilute solution of hydrochloric acid before use (if left too long in the acid, the tempered steel spring is weakened and will break when depressed). The removal of the cadmium or zinc coating prevents any plating of this metal from the anode clips onto the artifact. For this same reason, copper clips should not be used. Copper clips on the positive terminal, like the exposed copper wire attached to the anode clips, eventually go into anodic dissolution when submerged in the electrolyte and plate onto the cathode. In order to prevent anodic dissolution, any copper wire that is exposed in the electrolyte should be coated with an acrylic, polyvinyl acetate, or silicone rubber.
For electrolytic cleaning of iron, 16-gauge expanded mild steel mesh with half-inch openings is an inexpensive but efficient anode material. This steel mesh is easily cut, relatively flexible, easy to form-fit around the artifact and does not conceal the artifact from view. It also permits free circulation of the electrolyte and will not trap any gases. Mild steel plates, and even sheets cut from mild steel drums, make serviceable and cheap anode material, but their rigidity makes them difficult to form-fit around an artifact. Specially constructed mild steel vats, or even 55-gallon steel drums, can serve both as the electrolytic vat and the anode material. When a form-fitted anode is desired, however, only expanded steel mesh is cheap enough and flexible enough for regular use.
Mild steel anodes are surprisingly durable. As long as an adequate alkaline pH (minimum of 8.5) is maintained at the surface of the mild steel anode, it is less susceptible to chloride corrosion and will even outlast stainless steel. To maintain this level of alkalinity, it may be necessary to circulate the electrolyte. Mild steel anodes are commonly used as the oxygen-hydrogen cell electrodes (Worth Carlin 1975, personal communication).
It is claimed that stainless steel makes a superior anode, and it is often recommended because it is relatively inert and seldom needs to be replaced. All stainless steels, however, do not make equally suitable anode material. A stainless steel with a high percentage of chromium and nickel or even titanium must be selected; Type 316 stainless steel, which is composed of 16-18 percent chromium, 10-14 percent nickel, and 2-3 percent molybdenum, is recommended. Only Type 316 stainless steel resists chloride corrosion and is a good alternative to mild steel anodes in an alkaline electrolyte.
The high cost of stainless steel generally restricts its use as anode material for large objects and makes it impractical to cut and form-fit it to clean a single artifact. The most practical way to use stainless steel anodes is in certain electrolytic setup alternatives that allow for a number of artifacts to be treated at one time. These setups will be described below.
As long as the hydroxyl ion concentration in the electrolyte is kept high, mild steel anodes are more efficient than stainless steel anodes. Regardless of the anode material, it is much simpler and more economical to change the electrolyte before the chlorides build up to such an extent that they alter the pH and electrode potential at the anode, destroying the anode's passivity and causing it to go into dissolution. When this happens, the anode has to be replaced.
A wide variety of containers can be used in an electrolytic setup. Many kinds of non-conductive vats of various caustic and acid-resistant plastics, such as polyvinyl chloride (PVC), polypropylene (PP), and polyethylene (PE) are widely used. PVC plastic pipes with sealed ends make excellent vats for long, slim artifacts, such as rifle barrels. Fiber-resistant plastics should be avoided unless it is certain that they are alkali-resistant. Glass containers are also suitable for electrolytic setups, as are wooden vats, or frames lined with sheets of PVC plastic (care must be taken not to puncture the plastic).
In addition to non-conducting containers, conducting mild steel vats are frequently used in electrolyte cleaning. The metal vat serves as all or part of the anode and may be substituted as such in any of the electrolytic setups described below. Metal vats also have a distinct advantage over plastic vats in that all stages of the conservation process can be carried out in them. This is especially advantageous for very large pieces, where it is not economically feasible to have different vats for electrolysis, rinsing/dehydration, and wax impregnation.
Mild steel vats can be constructed in various gauges and are surprisingly durable and versatile, even in the lighter gauges. Use a gauge that provides the strength required and does not increase the weight of the setup beyond the laboratory's ability to handle it. A common coffee can, with the can used as the vat and the anode, is a simple and effective container for small artifacts. Mild steel 55-gallon drums, cut lengthwise or in half, make readily available, cheap vats, which can be employed in any of the described setup alternatives, in combination with auxiliary anodes to assure a more even distribution of current. Welded mild steel vats can be constructed cheaply and will last for years. For very large artifacts, such as anchors, a two-piece 5-m long mild steel vat is recommended. This 'T'-shaped vat is constructed of two parts, the stem and the cross, each of which is open at one end. When the parts are joined, the vat is used to clean anchors with auxiliary sheets of expanded mild steel positioned near the top surface of the anchor in order to achieve a better distribution of current. Separated, the two vats can be employed to clean an assortment of large iron artifacts.
When corrosion takes place in a conducting mild steel vat, it will occur at the stress points, such as weld lines and bends in the metal. It is for this reason that North (1987:225) discourages the use of a mild steel vat as an anode. If a metal vat is not hooked up as the anode, the fact that it will not be anodically passivated affords it some protection; however, any chloride ions present will still eventually corrode the vat. Furthermore, if a mild steel vat completely corrodes in 10 years due to its use as the anode, it can be easily replaced many times and still be more economical than stainless steel or plastic alternatives. The recommendation by North is disregarded by most conservators responsible for treating large iron artifacts from marine sites.
Care must be taken to ensure that metal anode vats remain passive during electrolysis; otherwise, the metal will go into anodic dissolution and create perforations, which are difficult to repair. This is sometimes difficult when using low current density electrolysis in the presence of high levels of Cl- ions; however most of the difficulties can be overcome if a 5 percent sodium hydroxide electrolyte is used until the chloride levels decrease or the current density can be increased to keep the anode passive. This issue will be discussed in more detail later in this section.
Conservators should be aware of some safety issues regarding the use of metal vats as both the container and the anode. Most DC power supplies used in electrolytic cleaning operate in a 6- to 12- or a 24- to 32-volt range and a 0- to 50- or 0- to 200-amperage range, but the actual voltage used during electrolytic cleaning usually does not exceed 6 volts. At this voltage, there is little personal danger in using metal vats. In general, a voltage of less than 32 volts is not hazardous because the IR drop in the human body is such that little or no current would pass through the body. Care should be taken, however, to avoid shorting the two terminals of higher voltage power supplies against each other.
Chloride concentration monitoring of the electrolyte is crucial to the efficiency and success of electrolytic reduction when conserving metal objects recovered from marine environments. Several methods to quantitatively monitor chloride concentration exist. The mercuric nitrate titration method is recommended in this manual due to its simplicity and low cost.
MERCURIC NITRATE METHOD OF CHLORIDE DETERMINATION
The mercuric nitrate test is a quantitative method used to determine Cl- or NaCl in parts per million in an aqueous solution. It is a quick and simple test to perform and gives accurate and consistent results. The following procedure is a modification of the method outlined by Furman (1962:331-332).
1. One automatic 25 ml burette
2. Two small amber glass bottles with droppers
3. One 500 ml amber glass bottle
4. One 250 ml beaker
5. A magnetic stirrer
6. Teflon-coated stirring bars
1. Diphenylcarbazone-bromophenol blue indicator
2. 0.02N mercuric nitrate solution.
3. Sulfuric acid
Transfer the blue indicator and the sulfuric acid to the amber dropper bottles. The mercuric nitrate should be stored in the amber glass bottle for refilling the burette.
1. Take a 20 ml sample of electrolyte or solution to be tested, and place it in the glass beaker.
2. Place the beaker on the magnetic stirrer and put a Teflon stirring bar in the beaker.
3. Adjust the stirrer until the liquid is in a steady swirl.
4. Add five drops of diphenylcarbazone-bromophenol blue indicator. This will change the color of the solution to blue.
5. Add drops of sulfuric acid (usually 18N for sodium hydroxide electrolytes, 9N for sodium carbonate electrolytes, or 4.5N for water solutions) to the solution until an acid end point is reached. The acid end point is indicated by a color change from blue to clear. (The amount of sulfuric acid does not need to be measured as it only acidifies the sample for the next step.)
6. Titrate 0.02N mercuric nitrate, drop by drop, from the automatic burette into the beaker until the solution reaches a violet end point. The color changes gradually from clear to violet. Near the end point, each drop will show a flash of color. Continue until a single drops swirls into a single even violet color through out the solution. Note: the sensitivity of the titration can be increased by using a smaller normality solution or decreased by using a larger normality solution of mercuric nitrate.
7. Note the amount of mercuric nitrate titrated to reach the end point.
The concentration of chloride or sodium chloride in parts per million is calculated by the following formulas:
T x N x 0.03545 x 1,000,000
|= T x N x 1772.5 = ppm Cl-|
T x N x 0.05846 x 1,000,000
|= T x N x 2923 = ppm NaCl|
T = amount of mercuric nitrate titrated
N = normality of mercuric nitrate
In order to facilitate the calculation of chloride concentrations, a conversion table can be established using the formulas above:
|Amount of Mercuric Nitrate Titrated (ml)||ppm Cl-||ppm NaCl|
The mercuric nitrate test gives the total amount of Cl- or NaCl in the electrolyte. Unused electrolyte solution, however, will already contain a certain amount of chlorides. In order to determine the amount of chlorides expelled from the artifact, the amount of chlorides present in an unused sample of the electrolyte must be determined. This provides a 'blank' which is subtracted from the amount of chlorides present in a sample taken from the electrolytic bath. For example, if a sample taken from an active electrolytic bath contains 24.5 ppm of Cl- while an unused sample of the same electrolyte contains 17.5 ppm, the amount of chlorides in the electrolytic bath that have been expelled from the artifact is 7.0 ppm.
Comments on the Mercuric Nitrate Test
Throughout this entire process, the glassware must be kept clean and uncontaminated. To prevent cross contamination, a clean beaker and stirring rod should be used for each electrolyte sample, or these items should be washed thoroughly and rinsed with de-ionized water between each sample. Purple mercuric nitrate stains can be removed from the stirring rods by immersing them in dilute solution of nitric acid.
Two notes should be added with regard to the chloride testing procedure. First, the diphenylcarbazone-bromophenol end point is to some degree subjective, but most individuals are consistent about their end point. Therefore, the most reliable and consistent results are obtained when only one individual is responsible for monitoring the electrolyte. To further assure the consistency of chloride monitoring, the reagents should be tested weekly against a known sodium chloride solution.
All of the chemicals in the concentrations required for the chloride test can be purchased from a chemical supply house. Chemical costs can be considerably decreased, however, if the chemicals are mixed in the lab as follows:
18N Sulfuric Acid, H2SO4: Dilute reagent-grade sulfuric acid with an equal volume of distilled or de-ionized water. Slowly add the acid to the water, never the water to the acid. Extreme heat will be generated. Let cool.
0.02N Mercuric Nitrate Solution, Hg(NO3)2 · H2O: Dissolve 3.42 g of reagent-grade mercuric nitrate in 1 liter of distilled or de-ionized water.
Diphenylcarbazone-bromophenol blue indicator: Dissolve 0.5 g of reagent-grade crystalline diphenylcarbazone and 0.05 g of crystalline bromophenol blue in 100 ml of 95 percent ethanol.
1000 ppm Sodium chloride solution (to test reagents): Dissolve 1 g of reagent-grade sodium chloride in 1 liter of distilled or de-ionized water. Dilute in half for 500 ppm NaCl, dilute a second time for 250 ppm, etc.
During electrolytic cleaning, the chloride level should be calculated and recorded at least once a week. These calculations can be used to make a graph that visually depicts at a glance the progress of chloride removal from an artifact. This will save much valuable time and enable the conservator to determine when all detectable soluble chlorides have been removed from the object, as well as when to change a chloride-contaminated electrolyte. Systematic chloride monitoring assures that the artifact will remain in electrolysis for the least amount of time necessary. Chloride monitoring cannot, however, be used to determine the efficiency of the reduction of the iron compounds. This can only be confirmed with analytical tests on samples taken from the artifact before and after treatment.
The graph in Figure 10A.1 presents the progress of a typical iron artifact recovered from a marine environment. It clearly depicts the high initial rise in chlorides released from the artifact and the subsequent decrease as the electrolyte is periodically changed. Drops in the graph to the zero line represent electrolyte changes. The two-week gap in the graph in November indicates when the artifact was taken out of electrolysis in order to mechanically clean the remaining encrustation and loose corrosion products. Occasionally, a drop from a previous high is seen on some chloride graphs, especially when the chloride level is high. This may be caused by chlorides reacting with corrosion products from the anode, chlorate formation, or the actual liberation of chlorine which causes the chlorides to be undetectable by taking them out of solution. Electrolysis is continued until the chloride concentration levels off for several days and does not increase above that present in a 'blank' of the electrolyte.
Figure 10A.2. Graph depicting the diffusion of chlorides into solution during electrolysis.