Shop Talk: Practical Information for Finishers Pickling & Acid Dipping

By Dr. James H. Lindsay, Jr., AESF Fellow, Contributing Technical Editor

This is the seventh in a series of re- views looking back on the 25 year- old series,“AES Update,” begun by the late Dr. Donald Swalheim, and continued by others. These excerpts remind us how much has gone on before and how relevant it still re- mains. Though much has changed in the metal finishing industry, a large part of the old stuff is still rel- evant. As always, I may occasion- ally add my own words or com- ments in brackets [ ], just to put things in perspective. The primary goal is to remain faithful to the original article. This issue, you are looking at an article written by Lawrence J. Durney, on the subject of scale and oxide removal from metals by pick- ling and acid dipping. The prepara- tory steps in plating are the ones that are taken for granted, but if we mess up here, the rest of the process remove residues such as silicates, tion (3) is preferred, with the oxide might as well be forgotten. As which emanate from a foregoing op- being removed. However, direct reac- Durney showed, it is critical to do eration such as cleaning. tion with the metal (2) inevitably oc- things right the first time ... and at “Acid dip baths consist of weak-to- curs at local areas where there is no the very beginning. moderate acids which are used chiefly scale or where scale is removed first. to adjust pH before using acid plating The accompanying table shows the “On the subject of scale and oxide re- solutions. But, they also find utility in various chemistries used for the more moval from metals, some platers tend the removal of very light rust or ox- common substrates.] to use acid pickle and acid dip inter- ides that develop as a result of long changeably, and frequently refer to a transfer times in processing. Troubleshooting Acid Operations descaling operation as a pickle. “Under ideal conditions, every part Oil or Tar on Top of Acid However, these terms should be dis- to be plated is subjected to a specially “The cause of oil or tar formation on tinguished. selected acid step or series of steps in top of acid solutions almost always “Descaling acids are strong acid a cycle, with the objective of achiev- occurs before the part reaches the acid dips that remove heat-treating, cast- ing optimum results. In a practical step. Check for incomplete cleaning ing, hot-rolling or forging scales, or sense, this is seldom possible and and dragged-in soap from a previous heavy rust deposits. Generally, they compromise is necessary. Because of cleaner. Remember that some soaps are not used as part of a plating cycle, the wide variety of substrate materi- will set if the rinsewater is too cold, but rather as a pretreatment prior to als, shapes, equipment, soils, finishes, and it follows that warming the rinse some other finishing step. Unless etc., the number of potential cycles is after the cleaner may get rid of the descaling acids are specially inhib- almost limitless. For trouble-shooting problem. The oil and tar may some- ited, they usually remove appreciable purposes, however, the acid portions times be eliminated by using wetting amounts of base metal too. of all the cycles share some common agents, but this is a crutch and should “Acid pickles are moderate-to- features and problems. be used only as a temporary expedient. strong acid solutions used to remove [The essential result, along with metallic smuts, light scales such as typical pickling reactions for plating, Rusting During Transfer spot-welding scale or light rust, and to is shown in the figure. The first reac- Or During Rinse activate the surface by removing ox- tion shows the acid neutralizing re- “Too long a transfer time, too weak ides generated by polishing and buff- sidual alkaline cleaner. The second an acid, too much time for post rins- ing, air oxidation during storage, etc. and third describe the reactions going ing, or too much dissolved metal in In addition, acid pickles are used to on at the metal surface. Ideally, reac- the acid solutions are the usual causes

12 PLATING & SURFACE FINISHING of rusting during transfers through the rinsing stage. It is sometimes helpful Acid Solutions for Some Metal Substrates to reduce the flow of water in the rinse, converting it into a weak acid Metal Substrate Common Acid Solutions1 dip which either removes rust or pre- Aluminum 25–50 v/o nitric acid vents its formation. Strengthening the Copper, copper alloys 5–15 v/o sulfuric or hydrochloric acid acid or making a new acid solution High-carbon steel Anodic treatment in 20–65 v/o sulfuric acid, usually corrects the problem if it is 150–400 ASF, 1 min due to other causes. Leaded bronze 5–25 v/o fluoboric acid Stainless steels 1–2 v/o sulfuric and 0.1 v/o hydrofluoric Smut Formation in Acid Dip acids, 1–2 min “If the acid is too strong for the alloy Steel 5–10 v/o sulfuric acid, 15 sec to 2 min2 being processed or if the acid is con- taminated with some other metal, Zinc die castings 0.25–0.75 v/o sulfuric acid, 20–30 sec smut will form on the alloy surfaces. 1 Determining which of these is the All solutions at room temperature. 2 ° ° cause requires some careful testing, A 3–6 v/o sulfuric acid solution heated to 70–80 C (158–176 F) is commonly but the addition of an inhibitor will used for removing heavy scale. sometimes correct either one or both Water Break After Acid Dip operation, or a carbon deposit left of the conditions. Inhibitors, which “Often a difficult problem to track from heat-treating, brazing, gas-weld- come in liquid and solid forms, reduce down, water-breaking after acid dip- ing, etc. attack of the metal substrate areas ping requires very careful observation “Usually, reverse-current cleaning where scale is removed first. They to determine exactly which of the eliminates non-conductive smuts, al- function by forming adsorbed films causes exists. Once pinpointed, how- though, in difficult cases, it may be on the clean metal surface. Inhibitors ever, the solution is usually simple. necessary to use high currents and/or are particularly effective if the con- The causes are inadequate cleaning, heavily chelated materials. Chelates taminating metal is copper. acid contaminated with oil or fatty also are usually good deflocculants acid from soap, acid containing in- and help to remove smut, both by Smut Formation in the Rinse compatible wetting agents and/or in- deflocculating or suspending it in so- “A common occurrence with steel hibitors, or oils held under metallic lution, and, perhaps, by dissolving a when brass or copper is processed on smuts on surfaces. small amount of the metal substrate, the same line is smut formation dur- “Common causes for the inability thus helping to remove smut from the ing the rinsing stage. Copper dis- of an acid solution to remove light surface. In some cases, it may even be solved from racks causes smut forma- scale or oxide film include the use of necessary to use an alkaline deruster tion on steel; this also frequently hap- too-weak an acid and the necessity for type of material, possibly even with pens with aluminum alloys when cop- an addition of activators such as a periodic-reverse current. per from racks or parts being pro- fluoride salt into the acid. It is impor- cessed dissolves in the acid. The acid tant to find out if the acid had been Conductive Smut is strong enough to hold metal in so- working properly prior to the identifi- “Conductive smuts, which are usually lution until it is diluted by rinsewater, cation of the problem, and if parts metallic, may consist of fine particles which precipitates copper [as a] smut. and/or scales are the same as existed from buffing or polishing operations. Eventually, however, a significant in- on previously processed parts. They may also occur as a result of crease in the contamination level will “Spot-welding scale, in particular, machining, drawing or stamping op- lead to smut formation on parts during frequently vary in their difficulty of erations. Metallic fines are a nuisance immersion in the acid tank. removal depending on the condition because they are often strongly at- “To correct the situation when steel of the wire as it goes to the spot- tracted to surfaces and may adhere and brass are being used on the same welder (wire source, lubricant, etc. - sufficiently to interfere with oil re- line, use an inhibitor in the acid. have they been changed?); the condi- moval in a contact area. Sometimes, When copper contamination from tions of welding (voltages, these fine-particle smuts arise from racks is a problem, strip the racks ev- amperages, dwell-time, etc. - have metal precipitation from contaminated ery cycle and use an inhibitor in the they been altered?); and the storage acid solutions. acid. Rectifying smut formation when conditions after welding and before “Conductive smuts, with the excep- processing aluminum calls for the use plating. tion of those resulting from contami- of titanium or stainless [steel] racks nation of the acid, usually do not re- and stripping at every cycle. Other Types of Smut spond to reverse-current cleaning or, good corrective measures for smut “There are two basic types of smut: for that matter, even to direct-current problems with aluminum are: dump- non-conductive and conductive. cleaning. Since they are metallic, con- ing acid more frequently or adding a ductive smuts are best erased by weak nitric-acid step to remove the Non-Conductive Smut strong acids. Metallic smuts on steel, deposit. [A chromate-type desmutter “Non-conductive smut usually con- for example, may require hydrochlo- was also suggested, but today’s trend sists of inorganic particles which may ric acid as strong as 75 percent by away from hexavalent chromium be dirt, abrasives from buffing com- volume. As mentioned, these metallic chemistry may preclude its use.] pounds, carbon, silicon or other alloy- smuts may adhere so strongly to the ing constituents brought to the surface surface that they prevent complete oil during an etching or severe pickling removal, which allows the oil to show

November 2000 13 up after the smut is removed. If this ling operations. The problems gener- current, the conditions are ripe for happens, a subsequent alkaline clean- ally stem from: an unwanted plating operation. For ing step may be necessary. example, iron from the inside of • Changes in one or more of the tubing can build up to a point in Cathodic Pickling bath constituents as a result of which it will plate out in the high- “How about cathodic pickling? Elec- oxidation at the anode Some wet- current-density areas; or trolytic pickling has been used for ting agents, for example, may oxi- • Dragged-in materials from other many years, yet no common consen- dize to become poorly soluble oils pre-cleaning steps that may un- sus has been expressed on how well it that rinse very slowly and leave a dergo reactions leading to prob- works. For some people, it has been a haze; or lems with an otherwise acceptable tremendous help; others have found • Contaminants that may actually acid. The wrong wetter in a cleaner the results so erratic that they question start to plate out. Because, in most may be dragged into the electrolytic the wisdom of using current in pick- cases, electrolytic pickles use direct pickle and cause problems in the pickle by oxidizing; or • Carbon anodes that break up. However, if lead anodes are used, the lead may become the contami- nant that is unintentionally plated out.

“It is safe to say that nobody can avoid all cathodic-pickle problems, but there are some guidelines which can make the job a lot easier. One rec- ommendation is to use only the sim- pler powdered acids, whenever pos- sible. These acids, which do not con- tain fluorides or other accelerators, permit the use of lead anodes instead of carbon anodes, which are more likely to break up. Only occasionally will enough lead dissolve to cause a plating-out problem. When using lead anodes, it is a good idea to avoid ex- cessive anode current densities. “A second guideline is to watch for iron build-up if there are appreciable blind areas with articles such as tub- ing, and to change the solution before trouble develops. Inhibited acids fa- cilitate the operation. “If fluoride-type acids must be used, circumvent any problems as much as possible by using the best- grade carbon anodes available, while keeping the current density low and skimming the surface of the tank of- ten. “Make sure that cleaners used be- fore the electrolytic pickle have com- patible wetting agents. Use the cleaner that does an effective job with the least amount of wetter possible. “After the cathodic pickle treat- ment, ensure good rinsing of the ar- ticle, paying careful attention to this phase of the operation. “Finally, a reverse-current electrocleaner following the electro- lytic pickling operation should be em- ployed if at all possible. This should be followed by a rinse and a dilute acid dip if the plating solution is an acid solution. Admittedly, this proce- dure makes for a rather long cycle, but many of the older plating ma- chines have room for these steps and it certainly helps to produce a better Free Details: Circle 108 on reader service card or visit www.aesf.org final piece of work.” P&SF 14 PLATING & SURFACE FINISHING