Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:email@example.com
Plating onto High Strength Steels and Leaded Alloys
Quality plating with good adhesion, no star dusting, no pitted deposits is the goal of all platers. Poor adhesion can result in complete failure of the device. Often the smallest blister in a plated deposit results in failure. pitted deposits often fail quickly and detract from the appearance..
Troublesome alloys include Kovar (R), Invar az), stainless steels, brass, leaded steel, leaded copper, leaded brass bronze, case-hardened steel and high strength steels all require special preparation treatments different from mild steel and copper alloys which do not contain lead. In addition the treatments are different for each of these materials, often requiring many different chemical treatments.
High strength, low alloy steels have posed problems for platers. In order to achieve good adhesion, an extensive number of process steps are required. Some of the steps involve strong acids and a low pH nickel strike. These acid preparation steps contribute to hydrogen embrittlement of the basis metals plated. Hydrogen embrittlement can be catastrophic when stresses are applied. It can also reduce fatigue life. It is difficult to remove hydrogen form electroless nickel plated high-strength steels because the deposit is amorphous (without crystal structure). Hydrogen relief takes place more easily when there are grain boundaries for the hydrogen to follow during the baking process. To release hydrogen from electroless nickel plated parts, slow ramping of the temperature should be done, and longer baking time at the final temperature is the preferred procedure. If the temperature is too high during the beginning of the heat cycle, blisters may result from the extreme pressure under the deposit, causing fractures of the basis metal or the plated deposit. It is, therefore, best to limit the amount of hydrogen exposed to the metal during chemical processing and plating.
An important advance in good adhesion has been made. Additionally, the same chemical treatment can be used for all the basis materials mentioned above. It does not contribute to hydrogen embrittlement.
R trade name of Westinghouse Electric Corp. R trade name of International Nickel
Pickling and conditioning
Most pickling and activating solutions are strong acids having pH levels near or below 0.5. Most high strength steels and stainless steel alloys are activated using strong acids, such as hydrochloric acid or hot sulfuric acid. Hot hydrochloric acid is often used for preparation /activation of Kovar and Invar. I suggest that there is a citrate based Activating/conditioning/ pickle that will prepare all the above mentioned metals for adherent, pit free plated deposits.
The typical Ph of the new pickle/conditioner solution is 3.2. It is not considered corrosive by D.O.T. standards. Altough the Citric acid pickle/conditioner solution is not as corrosive as other pickling acids, it rapidly removes rust and light scale, leaving a very reactive, clean surface ready for subsequent finishing. The citric acid solution can be utilized for many so-called difficult-to-plate metals.
Citric acid forms chelates with metals which change their solubility characteristics, usually making he metal soluble where normally it is insoluble in the form of its chloride, sulfate, hydroxide or other salt. This is an advantage for cleaning operations, since metals can be dissolved at less corrosive pH levels (1). Metal oxides are more easily removed using citrate-based solutions. There is less tendency to re-deposit metals by chemical replacement (immersion deposits) when citrates are present at the right pH. citric acid solution are subject to mold growth if it does not have an antimicrobial compound added to the solution. Adjusting the pH to 3.2-3.4 using ammonium hydroxide adds to the effectiveness of the treatment process. It is used at room temperature to 70 degrees C (158 degrees F) The Typical use temperature is 24 degrees C (75degrees F), 6-7 minutes. Waste treatment is easy.
The pickle/conditioner process was developed primarily to prepare 4130 and 4100 series as well as 4340 and 4300 series low alloy high-strength steels for electroless nickel and sulfamate nickel plating. For these alloys the adhesion results are excellent. The process for preparation is greatly simplified by eliminating steps normally included in the typical preparation process..
Alkaline cleaning or vapor degreasing is followed by mechanical surface treatments such as bead blasting, sand blasting, shot peening, vapor blasting or grinding. Grinding may leave metal splinters which should be removed by electropolishing or mechanical polishing. Further cleaning is required prior to plating. Anodic electrocleaning or periodic reverse, exiting on the anodic current cycle is preferred. Many different pickling and activation treatments have been used with some success in the past. Hot sulfuric acid, very strong hydrochloric acid and fluorides have been used, followed by a desmutting step. Another anodic clean, another acid treatment is often used. DO use inhibited acids if strong acids are to be used to minimize hydrogen embrittlement. DON'T forget to bake soon after plating.
The use of the citrate-based pickle/conditioner simplifies the procedure and assures good adhesion. A Woods nickel strike, or low pH sulfamate nickel strike is often required to promote adhesion of plated deposits to these HSLA steels.
The citrate process was tried experimentally with cast iron with equally satisfactory results. This lead to its use on case hardened steel and leaded steel, both of which resulted in good adhesion and good quality deposits. Using mineral acids such as hydrochloric acid or sulfuric acid to prepare leaded steels, and brass results in leaving insoluble compounds on the surface that result in star dusting when bright nickel plating is used_ lead sulfate and lead chloride are insoluble. Since soluble compounds of lead are formed by the citrate solution, it is possible to process these metals satisfactorily: In addition, it is suitable for processing copper, copper alloys, zinc and zinc alloys. The results were excellent, with good adhesion and no blisters ( including minute blisters of 5-100 micron meters) that often appear when these alloys are treated by conventional mineral acid preparation solutions. Alternate acids which produce good adhesion are fluoboric acid and sulfamic acid. However, fluoborates introduce waste treatment problems and sulfamic acid hydrolyzes to form sulfuric acid. Since lead appears in copper and brass castings as discrete globules, and in the casting process, lead is concentrated at or near the surface, the formation of insoluble salts can result in blistered plated deposits.
Electroplated nickel and many electroless nickel deposits quickly form oxide films which must be removed to assure adhesion of subsequent plated deposits. The longer the exposure to air, the more difficult it is to remove the oxide film and to keep oxides off long enough to produce quality plated deposits. Oxide films can and do form in rinse water. It is important to have short but adequate rinses after activation of nickel.
In the Laboratory, to our amazement, the citric acid solution allowed adherent electroless nickel and sulfamate nickel to be plated over electrodeposited sulfamate nickel, bright nickel, watts nickel and most electroless nickels without the use of a. low pH electronickel strike such as a Woods strike 240 g/L (32 oz/gal) nickel chloride in 10% hydrochloric acid) or a low pH activating sulfamate nickel strike, pH 1-L2.
The adhesion of nickel-on-nickel deposits has been tested by various tests: bend, grinding and bulge tests methods. It should be noted that when low pH nickel strike solutions of proper formulation that are free form contaminants, including iron, chromium, lead, etc., are used over nickel, nickel plated deposits and nickel alloys, provide excellent adhesion. When in doubt about the nickel alloy to be plated, use a proper low pH strike prior to subsequent plating. The low pH sulfamate strike is preferred because it is somewhat more tolerant to impurities, has some better throwing power and will work on a larger variety of alloys, The strike is made using nickel sulfamate solution with the nickel metal at 45 g/L to 75 g/L (6-10 oz/gal), the pH lowered to about 2 using sulfamic acid (very pure grade) then lowered further to pH 1-1.5 using Hydrochloric acid. 4 oz/gal boric acid is added to the solution. It is used at room temperature and at high current density.
The processing cycles and operating data shown are for typical applications and may have to be modified for specific applications. Factors influencing modified operation are: condition of the basis metal, including porosity, extreme roughness with entrapped foreign matter in the surface, metal splinters, etc., type and extent of soil(s) to be removed, part configuration, equipment and space considerations, and plating requirements.
HIGH-STRENGTH LOW-ALLOY STEELS
(HSLA 4130 and 4100 SERIES)
1. Soak clean
HIGH-STRENGTH LOW-ALLOY STEELS
(HSLA 4340 and 4300 SERIES)
CASE HARDENED STEEL
1. Anodic electroclean (reverse current)
2. Water rinse
3. Pickle/condition, 30% by volume, 49 deg. C (120 degrees F ), 2 minutes
4. Water rinse
5. 1/2% ammonium hydroxide dip if electroless nickel is used, Omit for other plates
LEADED STEEL (FREE MACHINING STEELS), OR LEADED BRASS
NICKEL AND NICKEL PLATED DEPOSITS
4. Water rinse
5. Plate with electroless nickel or electroplated nickel. Note: This cycle can be used for bumper-replate operations. Longer cleaning cycles to remove buffing compound are usually required for bumpers.
Cycle same as for Case Hardened Steel
Many process cycles for activation of Kovar include alkaline permanganate and mineral acid
steps.The Pickle/conditionersolution is used to replace these steps. The pickle/conditioner is used
at 15% by volume, 49 degrees C (120 degrees F) for 5 - 6 minutes
Portions of this paper were presented at EN 95, "Sponsored by Products Finishing" magazine.
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