Choosing the best electroless nickel process for the application...
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Anybody who has spent any time in or around the finishing industry has probably seen plating requirements for a part that leave a lot of unanswered questions. One of my personal favorites is the classic callout for "nickel plating" on an engineering drawing or purchase order. That's it, nothing else. Just "nickel plating." It's like entering Baskin-Robbins 31 Flavors® and ordering ice cream. Could you be a little more specific? Electroless? Electrolytic? Sulfamate? Watts? High-phos? Mid-phos? A poorly defined requirement may spark a heated debate over the actual type of nickel plating the engineer intended. For certain applications, more than one EN process may provide adequate results, but in most instances, there is clearly a best option.
Most part drawings have the plating requirements defined in the note section or a specification callout. The plating requirements are typically non-negotiable, unless the requirement seems outdated or inappropriate. However, when requirements are not clearly defined or are not the best available option, finishers must help the customer resolve any inconsistencies. Many loosely specified plating requirements still exist because of changes in technology or a lack of knowledge regarding available choices. As an industry, we need to educate and actively promote the most current information regarding EN.
Today the buzzwords "partnering" and "strategic alignment" have become a regular part of our vocabulary. But what do they really mean? As corporations strive towards vendor-base reduction, the need for improved communication between the design engineer and the finisher becomes even more critical. Suppliers are expected to provide more than just commodity services at the cheapest price. Although this greater level of communication places a bigger responsibility upon the finisher, it can also lead to greater rewards. Long-term relationships and/or contracts, input on finishing requirements at the design stage, preferential vendor status (approved supplier, etc.) and additional opportunities for vendors who consistently perform well are just a few examples.
Although it sounds amazingly simple, there are many instances where the right questions are never asked, and the result is a less-than-satisfied customer. Determining the best EN process can only be accomplished by asking the right questions upfront. Fully understanding the specific requirements will undoubtedly lead you down the right path toward recommending the best EN process to the customer: 1) What are the customer's expectations of the EN plating? 2) What are the primary and secondary functions of the coating? 3) What service environment is the plated component going to be used in? 4) What properties does the application require, and in what order of priority? 5) Is a specification or specific process appropriate, or is a lesser degree of detail sufficient? The answer to these and similar questions are essential to the success of the application.
Checklists are one of the best methods for gathering all the pertinent information. By using a checklist, nothing about the specifics of the application will be overlooked. Most often, the customer can complete it or someone from your staff or other non-technical person can prompt the customer along. Usually prototype and other R&D projects do not include prints or other written plating requirements. For these situations, it is especially important to gather as much information as possible about the part and the application. Some of the key information most frequently requested is included in the following sample.
Est. Annual Volume:
Masking Required? Y/N
Barrel vs. Rack:
Main objective of EN plating: (rate in order of importance)
___Uniform dimensional buildup
___Release properties/Low coefficient of friction
Current Plating Requirements:
Other coatings used/tested:
EN thickness requirements:
Service Environment Conditions:
Process Requested by Customer:
Additional application information:
A plating process is more than just dunking parts into a plating solution. The EN plating solution is only one component in a sequence of processing steps. Although the bath chemistry and composition are important elements of the EN plating process, other considerations are equally important to the success of the application. The best plating process is one that, after careful evaluation of all of the available application information, translates into a clearly defined sequence of preplating, plating, post-plating and/or testing processes. If done correctly, the result will be highly consistent EN plated components that perform to expectations at a cost-effective price. Once the process has proved successful, the specifics should be captured either on the drawing or in a procedure to ensure repeatability.
Stress Relief. Always consider the necessity for preplate stress relief on hardened steel components. Depending upon the alloy, hardness (ultimate tensile strength), manufacturing practices and the end use, preplate stress relief may be recommended to reduce and/or redistribute localized residual stresses left behind from machining, forming, welding, heat treatment or other manufacturing processes. All of the major EN specifications make reference to stress relief treatments prior to EN plating. Although you may not be plating to a specification, it is usually prudent to incorporate stress relief into the EN process for hardened steel components. Specific time and temperature guidelines can be obtained from industry specifications such as AMS 2404, AMS 2405, MIL-C-26074, and ASTM B-733.
Mechanical finishing operations help obtain an acceptable surface finish or remove gross surface contamination such as mill scale or weld slag. Vibratory deburring, blasting and tumble finishing techniques improve the surface condition of the unplated component and can significantly enhance the performance of EN plating. Shot-peening techniques are frequently used to redistribute localized stresses resulting from machining and fabricating processes.
Know your strengths and weaknesses! Are the components made from unusual or difficult-to-plate alloys? If so, highly specialized pretreatments may be necessary to ensure adequate initiation, adhesion and overall EN deposit quality. Often special activation processes, electrolytic strikes or immersion preplate deposits may be required to obtain acceptable results. Without the most appropriate pretreatment and the proper use of it, there is a good chance that the results will fall short of your customer's expectations. Usually, the negative effects of questionable pretreatment practices far outweigh short-term gains. Most often, it is better to refer those jobs to vendors that have adequate capabilities and experience. Unfortunately, even a single bad experience may unjustifiably lead to a negative opinion of EN coatings, even though the true problem is often a vendor-related incident.
Entire volumes of information and related data have been published regarding the properties and applications of EN. Rather than embark upon a deep discussion of the specific EN bath and deposit properties, this paper emphasizes the importance of being technically familiar with these topics. All of the major EN chemistry manufacturers have "hybrid" chemistry formulations that produce deposits possessing unique properties. EN chemistry manufacturers supply product information and technical advice to determine the best EN chemistry for each application.
Most EN chemistry manufacturers have a few all-purpose "workhorse" plating chemistries that can adequately accomplish the primary and secondary EN plating objectives. For example, mid-phosphorus plating chemistries produce EN deposits with adequate hardness and corrosion resistance, along with EN's well known uniformity. However, in other applications, an all-purpose-EN may not meet the intent of the design engineer. This one-size-fits-all approach to EN plating may overlook some of the better options available.
Some finishers offer only one or two EN processes, and many only plate certain types of alloys. While there is nothing inherently wrong with this approach to EN plating, it limits the choices available to the customer. To become a full service EN provider, more options must be available for consideration and testing. Although it is impractical to offer dozens of EN processes, a carefully selected handful of hybrid EN formulations, used in conjunction with the appropriate pretreatment processes, will equip a finisher for almost any EN plating requirement.
Hydrogen embrittlement relief. Hydrogen embrittlement relief after EN plating should be considered for all hardened ferrous alloys. As is the case with stress relief, all of the major specifications make reference to hydrogen embrittlement relief, especially for highly hardened steel components. Following good finishing practice, hardened steel components should be properly baked to remove the potentially detrimental effects of hydrogen absorbed during the pretreatment and EN plating processes. Possible exceptions to this rule of thumb could include parts that are not subject to extreme service conditions or parts that would be adversely affected by the temperatures attained during hydrogen embrittlement relief.
Some applications may benefit from a post-plate baking process to improve the adhesion of the EN deposit. Certain aluminum alloys, high-carbon steels and other alloys demonstrate substantial improvements in adhesion when baked after plating. Baking reduces EN deposit stresses, which can be detrimental to the success of the application. In some instances, the as-plated EN deposit is unstable and prone to localized blistering or other failure if tested before baking. The same parts exhibit excellent adhesion after baking. Time and temperature guidelines for baking to improve adhesion can be found in the previously mentioned EN industry specifications.
EN deposits have the unique ability to be age-hardened when heat-treated at elevated temperatures (490F+). Hardness values in excess of 68 Rc can be obtained from carefully specified heat treatment processes. The downside to heat treatment is a reduction in the corrosion resistance, especially on high-phosphorus deposits. Heat treatment of EN deposits facilitates the conversion of the nickel-phosphorus alloy from an amorphous structure to a crystalline structure. Caution must be exercised when specifying a heat treatment process so as not to soften the substrate while hardening the EN deposit. Many hardness applications have converted to low- or low-mid-phosphorus chemistries, which can attain similar hardness values right out of the plating bath, eliminating the need for heat treatment.
Chromate treatments are sometimes used to improve the corrosion resistance of certain EN-plated components. Aluminum and iron castings, powdered metal parts and other porous components are frequently immersed in dilute chromic acid solutions after EN plating to passivate the EN deposit and seal exposed pores in the coating. Other post-plate rinse solutions contain surfactants and wetting agents that aid drying and reduce staining in cosmetic applications.
Did the EN process achieve the anticipated results? Meaningful testing and validation methods are key to ensuring deposit quality and performance standards. Testing EN deposits is typically divided into two categories.
Acceptance tests are used to validate conformance of the EN deposit on an actual group of plated components. Acceptability is determined using some or all of the elements listed above, based on which criteria are deemed necessary to the application's success. For example, the hydrogen embrittlement test would probably not be recommended for a soft, mild steel component.
Qualification tests are used to validate a particular EN process. Again, acceptability is determined using some or all of the elements listed above. Qualification tests are geared towards theoretical capability of the EN process, whereas acceptance tests are geared towards workmanship and actual deposit characteristics.
All of the major EN industry specifications provide guidance on testing methods, and most of the basic acceptance tests do not add cost to the process. In contrast, the qualification tests can and often do add additional cost to the EN process. Meaningful qualification tests are indicators of EN process capability and should be performed at the onset of a new program. However, to avoid additional expense, only tests relevant to the success of the application should be specified. After the initial qualification, testing should be performed at a frequency that ensures continuous process conformance.
Plating and testing actual parts is extremely important to validating the success of EN plating. After gathering all of the application information and prescribing a carefully selected EN process, there is no better way to determine success than by prototyping on actual production parts. The proof is in the pudding! Simulating service conditions can be accomplished using Falex® wear resistance testing, Taber® abrasion resistance testing, accelerated corrosion resistance testing and others. Whenever possible, actual in-the-field testing is recommended. Theory is great, but data is better.
Modifications in chemistry or thickness can be made after testing to fine tune the process and deliver the best possible EN finish.
None of the topics presented in this paper is brand new, cutting-edge technology. You have probably been exposed to most of the information presented. Successful EN plating requires more than just dipping parts in any old EN plating solution. Industry demands have placed a bigger burden upon the finishing community to provide high-performance EN coatings that consistently meet the expectations of the customer . . . the first time and every time. Gathering the basic information, reviewing the application and matching up the requirements with the best pretreatment, plating and post-plating options can only accomplish this. By taking a more "holistic" approach toward the entire EN process, the chances for success increase greatly. Ultimately, everyone wins when the result is The Best EN Process for the Application.