I did mean sodium hydroxide (lye).
Some time ago, I read everything I could find on the web regarding electrolytic cleaning. Alot of information was on preservation of iron artifacts recovered from marine sites, so what they are trying to accomplish is more complicated than blasting some rust and grease from a pan.
The information on electrolytes below came from preservation research at Texas A&M. I have highlighted some of the more important points. I have been using only sodium hydroxide for about 2 years now. Since it is a very dilute solution, I am not overly concerned with safety as it is much less aggressive than the lye solution I use for removing grease.
Jeff
ELECTROLYTES
The only two electrolytes commonly used in conservation for treatment of iron objects are alkaline solutions of sodium carbonate (Na2CO3) and sodium hydroxide (NaOH), the most alkaline electrolyte one can get. In all cases it should be kept in mind that the alkalies (and acids) used in conservation should be concentrated enough to do the required job, but no stronger. This avoids over cleaning the specimen and helps keep the operating cost as low as possible. In recent years there has been a switch from a general use of sodium hydroxide to sodium carbonate. In his first description of electrolytic reduction, Plenderleith (1956:194) recommended a 5% solution of sodium hydroxide, but in the recent revision of the book, Plenderleith and Werner (1971:198), only a 5% sodium carbonate solution is recommended. No discussion is presented as to the advantages and disadvantages of each electrolyte and under what circumstances one is preferable over the other. In proper conservation procedures, both electrolytes have their uses and the conservator needs to know when to choose the one best suited for the object at hand.
Sodium Carbonate
Generally speaking, a 5% sodium carbonate electrolyte with a pH of 11.5 will suffice for the electrolytic cleaning of most iron artifacts if maximum reduction is not the objective. In terms of safety, it is much less caustic than sodium hydroxide and is much safer to handle. It is less conductive than sodium hydroxide and has to be used in stronger concentrations, 5 to 10% versus 2 to 5%. It is less soluble, which makes it more difficult to mix, but it does not generate the extreme heat of sodium hydroxide when mixed in concentrated solutions. When expense is considered, the stronger percentages of sodium carbonate are only slightly less costly, and sodium carbonate is usually easier to obtain from chemical supply houses than sodium hydroxide.
Preliminary experiments comparing 5% sodium carbonate electrolyte to 2% sodium hydroxide produced some interesting data. One set of experiments (Locke, ms) compared artifacts treated in 5% sodium carbonate mixed in tap and deionized water and 2% sodium hydroxide mixed in tap and deionized water. In all cases chlorides seem to diffuse out of the artifacts and reach higher Cl- concentration in the sodium carbonate electrolyte more quickly than comparable artifacts in sodium hydroxide electrolyte. The major problem encountered with the sodium carbonate is the cathodic precipitates of insoluble carbonate on artifacts. The carbonate precipitates from the electrolyte and is more apt to happen at high current densities and when certain tap water is used to mix the electrolyte.
The marked carbonate deposit on the artifacts treated in 5% Na2CO3 is much more prevalent if the tap water has a large amount of carbonate in it. Artifacts from salt water, encrusted with calcium carbonate, magnesium hydroxide, and other minerals can provide the necessary elements to react with the carbonate in the electrolyte to form insoluble carbonates such as calcium or possibly magnesium carbonate. Once an article is plated with a carbonate deposit, it can seal off the surface and chlorides can be trapped inside, misleading the conservator as to when treatment of the artifact has been completed.
If a white, insoluble carbonate precipitate occurs when using sodium carbonate as the electrolyte, then gluconic acid, sodium gluconate or sodium glucoheptanate can be added as a sequestering agent to the electrolyte. With the addition of gluconic acid there was a decrease in the deposit, but it was still very apparent on iron objects in Na2CO3 in tap water, and slightly noticeable in Na2CO3 in deionized water. By adding 2% of the weight of the sodium carbonate in the electrolyte as gluconic acid or sodium gluconate, the tendency to deposit carbonate is reduced but not always eliminated. For maximum effectiveness gluconic acid or sodium gluconate require an excess of a free base (hydroxide). The pH of 5% sodium carbonate (11.5) is too low for maximum effectiveness. Tests with sodium glucoheptanate as a sequesterant appears to eliminate the carbonate deposit. If the tap water in an area contributed to the formation on the precipitate, then only deionized water should be used with Na2CO3. In the precipitate is not noticed, then there is no reason to go to the trouble of adding a sequesterent to the electrolyte.
If a carbonate deposit does precipitate on an object it is usually impossible to brush off or remove by electrolysis. The carbonate deposit can, however, be eliminated by soaking several days in a 5% solution of sodium sesquicarbonate or sodium hexametaphosphate. Sodium sesquicarbonate complexes with the insoluble calcium or magnesium salts to form soluble salts in the same manner as sodium hexametaphosphates (Plenderleith and Werner 1971:253).
Another major problem commonly encountered with a sodium carbonate electrolyte is that the pH and conductivity of the solution are not adequate to keep the mild steel anodes passive in the presence of high chloride levels. In sodium carbonate (OH)- ions discharge as oxygen at the anode more readily than the dissociation products of carbonate ions (CO3)-2. The anode becomes acidic by the accumulation of hydrogen from the oxygen evolution reaction; therefore, anodic dissolution is more prevalent than when NaOH, with its surplus of hydroxyl ions, is used. To avoid this the mild steel anodes must be cleaned and replaced more often than in sodium hydroxide, especially if the electrolyte is not circulated. It has also been noted that takes longer to rinse out all residue of a sodium carbonate electrolyte than to it does to rinse out all residue of a sodium hydroxide electrolyte.
The most important disadvantage of Na2CO3 relates to its pH and cathode reduction potentials. Theoretically, more efficient reduction of a ferrous corrosion compound is possible using 2% to 5% NaOH with a pH of 12.9 than 5% Na2CO3 with a pH of 11.5. More details in regard to this are discussed subsequently under electrode potentials. In general, sodium carbonate can be used as an electrolyte if reduction is not the objective, but when it come to treating metals from marine site, sodium hydroxide is preferred.
Sodium Hydroxide
The shortcomings of 5% Na2CO3 are overcome by using 2 to 5% NaOH with its higher pH. However, this electrolyte being much more caustic, constantly presents a potential hazard to those working with it. Caution has to be exercised, and adequate safety equipment such as gloves, eye shields, eye washes, and safety showers should be available. In spite of these problems, a 2% solution of NaOH is the only option is the objective is to reduce corrosion products. In most laboratories that treat iron recovered from marine sites, it the standard electrolyte.
