Good adhesion is not possible without a proper process cycle. Process cycles for a variety of metal substrates are given…
These strategies are accumulated based upon the success they provided though the experience of many. They are intended to be used as a base line or starting point strategy when a particular substrate is going to be processed. It is important to keep this in mind, and, yes, there likely will be disagreement with some cycle approaches. It is the belief of these authors this is good, because from disagreement comes new ideas and approaches. Another good source of information on cycle strategies can be found with in the ASTM B08 Committee. The ASTM Annual Book of Standards, Volume 02.05, "Metallic and Inorganic Coatings: Metal Powders, Sintered P/M Structural Parts" is a necessity for review regarding process cycles.
Carbon steels, low and high carbon, with and without heat scale. Most process cycles, even poorly designed ones, have the capability to process low carbon steel successfully. However, much of the success will depend upon choosing the proper alkaline cleaner that will remove the widest variety of soils. These cycles normally are characterized by the use of highly alkaline soak cleaners and electrocleaners. Polarity for the electrocleaner can either be anodic (reverse), cathodic (direct) or periodic reverse (alternating reverse and direct). Most process lines are set up with anodic or periodic reverse capabilities. Cycles for low carbon steels use strong mineral acids, including hydrochloric, sulfuric or acid salt mixtures.
High-carbon steels use similar strategies except there is concern that substrate carbon smut can form as a result of the acid activation. Using minimal activation times in a more dilute acid is critical. Sometimes an anodic (electrified) sulfuric acid improves adhesion significantly. Normally, an additional anodic or periodic reverse electrocleaning is often necessary after the acid activation to remove smut. Checking the surface for smut after the second electroclean is recommended. A mild acid neutralization after the second electroclean will neutralize any cleaner films and is recommended. A post-plating bake of 250-350F for 1 hr promotes improved adhesion. When scale is present on the substrate, modifying the cycle order is important to ensure complete removal of the scale. After soak cleaning to remove oils, an acid pickle is used to remove the scale. After the scale removal is completed, it is important to reclean the substrate because there is likely some soil or oil residue underneath the scale. Removing the scale exposes more soil residue.
Scaled High-Carbon Steel
High-strength alloy steels, tool steels, stainless steels or monels. The surfaces of tool-quality steels and those containing nickel must be prepared carefully to avoid over cleaning or under activation of the surface. Periodic reverse or cathodic (direct) electrocleaning is recommended to minimize the formation of passive oxide films on the surfaces. It is important to remember that anodic electrocleaning produces oxygen at the part and may be detrimental to the success of proper activation. Sulfamate or modified Woods nickel strikes are recommended to activate the surface of the work. The high chloride Woods nickel strike is an effective activator (especially for 316 stainless steel), but dragin of chloride into the electroless nickel tank can cause brittle, highly stressed, less corrosion resistant deposits. Dragin of sulfamate or Watts components are less detrimental to the operation of the electroless nickel chemistry. An additional benefit of the sulfamate strike is that cathode efficiencies are three to four times higher than a Woods strike, resulting in faster, more complete activation (especially in barrel applications). Metallic impurities can be dummied easily with the sulfamate, modified Woods and Watts strike systems.
Cycles for high-strength low-alloy steel (HSLA) substrates can vary depending upon the actual alloy processed. For example, a 4340 alloy cycle will vary slightly from a 4130 alloy cycle because the 4300 series contains nickel. Therefore, a second alkaline electroclean after the acid pickle activation is not recommended because that will oxidize the nickel in the alloy and cause the potential for adhesion loss. In this case, using acids that do not attack the 4100 and 4300 alloy steels to form smut are recommended.
Alloys over 38 Rockwell C in hardness require a preplate stress relief bake at 200-400F prior to plating to improve adhesion. It is important not to exceed the temperature at which the steel is tempered. Additionally, a post bake at 250-350F within 4 hr after plating is necessary for hydrogen embrittlement relief. Shot peening is sometimes carried out on the substrate before plating to reduce the stress in the alloy. This step often improves adhesion. Anodic acids are sometimes used to maximize adhesion. Ordinarily a 15-35% v/v sulfuric acid solution at room temperature applied anodically at 100-200 asf for a maximum of 1 min is all that is required. The minimal activation time is required since the potential for smut formation is greater. If smut formation occurs, use anodic electrocleaning after the acid to remove it, but make sure smut does not form in the second electroclean.
HSLA 4130 and 4100 Series
HSLA 4340 and 4300 Series
General High-Strength Steels (No Smut)
General High-Strength Steels (Smut)
Stainless steel. All grades of stainless steel can be successfully plated with electroless nickel once the oxide is removed. Stainless steel readily forms a tenacious oxide film, and, while this oxide is what helps make it resistant to most environments, it presents a problem for finishers. A Woods nickel strike is recommended for 300 series stainless because it simultaneously activates the surface while providing a nickel displacement coating that helps improve adhesion. If hydrochloric acid is used prior to a Woods or modified Woods strike, no rinse is necessary between them. If there is no electrified acid activate in the process cycle, the polarity in the electrocleaner is best maintained cathodically. However, a cathodic electroclean must be kept "clean" and never operated anodically on other substrates (alloys) because there is a risk of substrate surface contamination. A cathodic only electrocleaner is best for captive jobs or captive shops and is often not practical for job shops.
Stainless Steel (Option 1) 1. High alkalinity soak clean at 170-190F for 5 min 2. Rinse 3. Alkaline electroclean (cathodic or periodic reverse ending cathodic) at 150-180F and 20-100 asf for 2 min 4. Rinse 5. Rinse 6. 50% HCl, 10% H2SO4 or proprietary acid salts for 2 min 7. Rinse 8. Rinse (optional if HCl is used) 9. Wood's or modified Wood's strike at 25-75 asf for 2-5 min 10. Rinse 11. Electroless nickel plate
Stainless Steel (Option 2) 1. High alkalinity soak clean at 170-190F for 5 min 2. Rinse 3. Alkaline electroclean (cathodic or periodic reverse ending cathodic) at 150-180F and 20-100 asf for 2 min 4. Rinse 5. Rinse 6. 10% H2SO4 or proprietary acid salts at 20-30 asf (anodic or cathodic) for 2 min 7. Rinse 8. Rinse 9. Wood's, modified Wood's or sulfamate nickel strike at 25-75 asf for 2-5 min 10. Rinse 11. Rinse 12. Electroless nickel plate
Stainless Steel CF8M and 316 alloy (Option 3) 1. High alkalinity soak clean at 170-190F for 5 min 2. Rinse 3. Alkaline electroclean (cathodic or periodic reverse ending cathodic) at 150-180F and 20-100 asf for 2 min 4. Rinse 5. Rinse 6. Anodic etch in 25% v/v sulfuric acid at 20C for 2 min (use chemically pure lead cathodes) 7. Allow parts to stand without current for 10?15 min in 25% v/v sulfuric acid 8. Rinse 9. Cathodically activate parts in a separate (second) 25% v/v sulfuric acid at 20C for 2 min (use chemically pure lead cathodes) 10. Rinse 11. Wood's, modified Wood's or sulfamate nickel strike at 25-75 asf for 2-5 min (use live entry) 12. Rinse 13. Electroless nickel plate
Nickel and Nickel Alloys Containing Cobalt 1. High alkalinity soak clean at 170-190F for 5 min 2. Rinse 3. Alkaline electroclean (cathodic or periodic reverse ending cathodic) at 150-180F and 20-100 asf for 2 min 4. Rinse 5. Rinse 6. 50% HCl, 10% H2SO4, acid salts or proprietary mix for 2 min 7. Rinse 8. Rinse 9. Wood's, modified Wood's or sulfamate nickel strike at 10-75 asf for 2-5 min 10. Rinse 11. Rinse 12. Electroless nickel plate
Castings including steel and powder metal. Electroless nickel plating of castings and powder metal is difficult because of the inherent porosity in these materials. The porosity is affected by the type of casting process and other mechanical finishing methods. Plating castings requires special precautions and considerations. Many times oils and other soils become entrapped in the porosity. The rinsing becomes critical. Additionally, the hot process cleaners used in the cleaner cycle tend to become entrapped in the substrate's porosity through a capillary action mechanism. Proper cleaner functionality will "wet out" or solubilize the soil from the porosity. When the part is removed from the hot cleaner and immersed into the room temperature or cooler rinse tank, the capillary or pore closes and entraps the solution salts, which includes the cleaner or soil. This occurrence can accelerate corrosion of the base material over time, cause poor adhesion of the nickel or cause bleed?out staining on the surface of the nickel. A hot rinse following the soak cleaner that is 10F hotter than the cleaner is recommended to alleviate these types of problems. It is important to cool the parts before the acid activate. Weaker alkaline or weaker acid surface preparation steps are recommended because casting surfaces tend to form smut more easily. The use of shorter acid cycle times helps reduce smut formation. Beware of "dirty castings," especially cast iron that may contain high amounts of oils and other impurities. Castings typically cut down on cleaner solution life.
Cast Iron and Powder Metal
Leaded alloys. Lead is added to improve a part's ability to be machined. The lead can cause a problem with the electroless nickel plating process. Lead at concentrations greater than 1.5 mg/L in the electroless nickel chemistry acts as a catalytic poison. The presence of smeared lead on the substrate surface during plating will likely result in poor adhesion or microscopic voids in the deposit. Lead can be removed from the substrate surface using a fluoride- or citrate-type acid activation. In some situations, sulfamic acid solutions or acid salts containing fluoride can be used, but they are not recommended over fluoboric acid or sulfamic/citrate mixes. Sulfuric acid should not be used as the acid activation since insoluble lead sulfates/sulfides may form on the surface. A sulfamate nickel, modified Woods or Watts strike can be used to improve the adhesion of properly activated leaded surfaces. Strong electrocleaner concentrations, higher alkalinity chemistries and high anodic current densities can cause "lead blooms" or higher concentrations of lead to be present on the surface. Care should be used for leaded brass substrates since more zinc can be removed from the surface (leaving a copper smut) during the anodic electrocleaning cycle. Sometimes cathodic electrocleaning is used to minimize substrate attack and minimize lead smears on the substrate surface interface.
Leaded Steel (No Scale)
Copper alloys. There are many types of copper substrates that require cleaning, but the cleaning strategy varies if the copper is buffed, not buffed or electroplated. Buffed copper requires a much different approach with soak cleaning. Specially designed soak cleaners operated at high temperatures are required to remove the buffing residues. Once the residues are removed, electrocleaning and acid activation will be the same as for non?buffed copper. Non?buffed copper and electroplated copper allow a more traditional approach with regard to cleaning and processing through the soak, electroclean, acid and plate. Generally, lower alkalinity cleaners are used at lower current densities. Anodic current is used for copper and cathodic current is used for brass. Sulfuric-based acid activators and acid salt mixtures are favored (except for leaded copper or brass) over hydrochloric acid. The success for copper depends upon the activation process used. Copper alloys will not catalytically initiate plating in the nickel phosphorus systems without an additional activation process after normal preparation. There are several options for catalyzing copper surfaces:
Copper and Copper Alloys
Aluminum alloys. The surface preparation of aluminum is especially critical due to the rapid formation of oxides that occur on the surface. The basic process cycle steps are similar to those used on other substrates but without electrocleaning since an aluminum pretreat cycle is an all immersion process. Also, due to the wide variability in alloy composition, physical surface conditions and soils present, the baseline cycles will almost certainly need to be tailored to fit the specific application. Process cycles for wrought alloys differ from cast aluminum cycles. More aggressive alkaline etching during the process cycle is preferred for most wrought alloys. Etching for cast alloys is kept to a minimum since silicon and other types of smut are more likely to occur due to reduced attack on the aluminum.
Deoxidization of the surface is necessary to improve adhesion. There are many types of deoxidizers , and the selection depends on the aluminum alloy type. High silicon containing alloys require fluorides to adequately deoxidize the surface. High copper containing alloys favor sulfate based chemistries for optimal deoxidization. A combination of nitric acid and fluoride salts is effective for deoxidizing most alloys. Chromated deoxidizers are not recommended on electroless nickel lines since chromium is a poison to electroless nickel solutions and can be the cause of adhesion problems. The most important choice for the aluminum process, especially when considering etches, is the type of desmutter/deoxidizer used, which is related to the alloy type:
All acid chemistries can be used, or a combination of alkaline?acid cycle is effective. All acid cycles have been shown to benefit cast aluminum alloys primarily due to the porosity that exists in those alloys. Acid cycles provide easier rinsing of the process solutions from the porosity in the casting. General soak cleaning with alkaline or acid non?etch cleaners is preferred even if the part is to be subjected to an etch cycle. The type of cleaner should be chosen based upon the ability to remove the soil. Etchants are not designed to be effective cleaners of oils and soils. Attempting to use them for this purpose may cause nonuniformly etched surfaces that result in dull streaking patterns and porous or pitted nickel deposits. Silicated cleaners should be avoided in the cycle especially if rinsing is not optimal due to the possibility of insoluble silicates precipitating on the surface of the work.
Zinc immersion displacement or zincate coating is applied to protect the surface of the aluminum from re?oxidizing until it is placed in the electroless bath. Some of the zincate coating is dissolved in the acid electroless nickel solution as deposition begins unless an alkaline electroless nickel strike is used. Double zincate cycles are used to maximize deposit adhesion and nickel deposit smoothness while minimizing the amount of zinc dissolved in solution. In some process situations for cast alloys, a single zincate process is favored over the traditional double zincate cycle. Alkaline electroless nickel strikes are used prior to the acid electroless nickel phosphorus chemistries to minimize this zincate dissolution and extend electroless nickel solution life. It can be shown that alkaline electroless nickel strike solutions provide cost savings to the finisher if used in the process line, which is the primary reason for their appeal.