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unless the requirement seems outdated or inappropriate.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.
Gathering Information
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.
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| EN can be used to plate a variety of parts used in a number of applications. |
Part Information
Part No.:
Material:
Hardness:
Est. Annual Volume:
Masking Required? Y/N
Barrel vs. Rack:
Application Information
Main objective of EN plating: (rate in order of importance)
___Corrosion Resistance
___Hardness/Wear
___Solder/Braze Application
___Uniform dimensional buildup
___Release properties/Low coefficient of friction
___Cosmetic Appearance
___Other:
Current Plating Requirements:
Other coatings used/tested:
EN thickness requirements:
Part Function:
Service Environment Conditions:
Process Requested by Customer:
Testing requirements/methods:
Additional application information:
Total EN Process
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.
Preplating Considerations
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 techniques. 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.
Chemical pretreatment. 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.
EN Plating Chemistry
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.
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| Plating specifications may be strict, such as those for parts, used in military
aircraft |
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.
Post-Plating Considerations
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.
Baking to improve adhesion. 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.
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| EN plated parts. |
Heat treatments to harden EN deposits. 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.
Chromating and other treatments. 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.
Testing for Results
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
Visual Appearance
Thickness
Adhesion
Porosity
Hydrogen Embrittlement
Uniformity |
Qualification Tests
Alloy Composition
Corrosion Resistance
Wear Resistance
Microhardness
Internal Stress |
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.
Prototyping and Other R&D
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.
REFERENCES:
1. AMS 2404D, Plating, Electroless Nickel, SAE International, 400 Commonwealth
Drive, Warrendale, PA 15096-0001
2. AMS 2405B, Electroless Nickel Plating, Low Phosphorus, SAE International,
400 Commonwealth Drive, Warrendale, PA 15096-0001
3. MIL-C-26074E, Military Specification, Coatings, Electroless Nickel, Requirements
for ASD/ENES, Wright Patterson AFB, OH 45433-6503
4. ASTM B 733-90, Standard Specification for Autocatalytic Nickel-Phosphorus
Coatings on Metals, ASTM Committee on Standards, 100 Barr Harbor Drive, West
Conshohocken, PA 19428
