If you are seeking to maintain uniformity in your plating application, Electroless Nickel Plating (ENP) is an excellent option to consider. This process allows you to maintain inside dimensions and irregular shapes for jobs that require uniform assembly tolerances.
Electroless nickel plating is an engineered finish that’s often used to improve corrosion resistance for many basis metals like aluminum, cuprous, and ferrous alloys. In this process, applied current is eliminated when plating auto-catalytic electroless nickel. It results in outstanding deposit uniformity even when it’s applied to the most complex shapes. Not only does this process provide great corrosion protection performance, but ENP is used to improve wear resistance, hardness, lubricity, as well as solderability and overall appearance of parts. The characteristics of the final finish is affected by the phosphorus content in the deposit. Post-plate heat treatments are often used to improve both the adhesion and hardness of electroless nickel plating.
MIL-C-26074 is the official specification for plating professionals who apply this high-precision coating. This specification is regularly used in the automotive, aerospace, power transmission & distribution, marine, rail, home electronics, oil & gas, and the pharmaceutical industries.
The MIL-C-26074 (AMS 26074) specification is used frequently in rack plating, barrel plating, and vibratory plating applications. Electroless nickel coatings are suitable for temperatures up to 1,000°F. However, at higher temperatures, the wear and corrosion resistance may experience some degradation.
Electroless nickel is similar in color to stainless steel and corrosion resistance and it’s good for coatings in excess of .001” in thickness. High phosphorous electroless nickel is a popular option for applications requiring increased corrosion resistance, for example in the food industry. This material is extremely hard as deposited, but it can be hardened by baking to achieve coats similar in hardness to hard chromium.
Even in relatively mild service applications, corrosion can cause functional issues and shortfalls in the final product. Electroless nickel plating provides an excellent solution to prevent corrosive attack in a range of different corrosive mechanisms, including galvanic corrosion, chemical attack and erosion.
Raw metals (not including precious metals) can readily oxidize and corrode over time when exposed to different environmental conditions and corrosive applications. Years of manufacturing experience has
found that the design of any component first needs to begin with a surface engineering evaluation to be sure that the product will function reliably over the intended service life.
Electroless Nickel Plating Applications
ENP plating is useful in a variety of base materials, including:
- Stainless steel
- Exotic metals like Inconel, Molybdenum, or Kovar Plating
ENP differs from electrolytic nickel plating because it doesn’t require an external source of electrons for deposition. In other words, this process does not need to pass electric current through parts to form the plated deposit. This gives the ENP deposit much better uniformity over electrolytic deposits. This improved uniformity enhances the corrosion resistance on all features of a part, especially traditionally difficult features like thru holes or counter bores. Another feature of ENP that makes it unique is that the nickel co-deposits phosphorus in varying amounts from 4-13% based on the bath type. The phosphorus level affects key deposit properties like ductility, hardness, and corrosion resistance. Also, post-plate heat treatment can be used to change the electroless nickel plating structure, which increases hardness by as much as 70 Rc.
Classes & Grades
There are four different classes and three grades of electroless nickel plating used throughout the industry. This is done to meet different specifications. The ENP class is the post-plating heat treatment process used, while the grade of an electroless nickel plating refers to its thickness.
Heat treatment after ENP tells metal plating professionals whether proper adhesion has happened, thereby improving the hardness of the crystalline structure of the amorphous deposit. There are six heat-treat classes available:
–Class 1 is used for coatings that require no heat treatment;
–Class 6 requires heat treatment of 300-320°C for at least an hour for improved coating adhesion; and
–Classes 2 through 5 are used for applications that fall between these two extremes.
Electroless nickel coatings are graded A, B, or C. Grade A requires a minimum thickness of 0.001”; Grade B requires a minimum thickness of 0.0005”; Grade C requires a minimum thickness of 0.0015”.
Types of ENP Reflect the Degree of Corrosion Protection
Traditional electroless nickel phosphorus plating is offered in three different categories. Based on design choices and the level of corrosion performance needed, phosphorus levels and deposit thickness can be adjusted. The three ENP types are explained in more detail below:
- High Phosphorus (11-13% P) – High phosphorus electroless nickel plating (high-phos) offers the best corrosion resistance. It’s non-magnetic, has the lowest melting point (~ 880 C) and it’s the most ductile of the EN deposits. The deposit has an amorphous structure with a hardness between 48-55 Rc as-plated. This can be increased to 66-70 Rc with post-plate heat treatment. Due to the amorphous structure, high phosphorus electroless nickel has the lowest as-plated internal stress making it the best electroless nickel for heavy build deposits capable of being plated to thicknesses of up to 0.005 inches per side. The appearance of the deposit tends to have a semi-bright look that varies depending on the raw surface finish of the plated part. High phosphorus electroless nickel has a slower plating rate as compared to medium phosphorus electroless nickel. This tends to increase the overall cost of this process.
- Medium Phosphorus (6-10% P) – Medium phosphorus electroless nickel (mid-phos) is considered to be the most popular electroless nickel type. It offers excellent corrosion resistance, a higher melting point (~1000 C) and it’s known for its solderability. Mid-phos ENPs in a mixed amorphous/micro crystalline state and has a hardness of 58-62 Rc as-plated, but it can be increased to 66-70 Rc with post-plate heat treatment options. This type has a semi-bright to bright appearance that varies depending on the raw surface finish of the part. A common reason for using this type is because of the faster plating rate that tends to reduce the cost of this process over high-phos EN.
- Low Phosphorus (5% or less P) – A low-phosphorus ENP (low-phos) type offers the highest as-plated hardness (up to 60 Rc). It’s the least ductile choice because of the micro-crystalline as-plated structure. Low-phos is the least corrosion resistant of the three electroless nickel types but it’s also the most conductive and solderable in the as-plated state. Today, there are not many metal finishing shops that offer the low-phos option because of more commercial demand for medium or high phosphorus varieties.
Heat Treatment of Electroless Nickel Plating
Many customers ask me how heat treatment affects deposit properties. Here’s a breakdown of the different heating options and the results produced:
1) High Temperature Postplate Bakes (500F-750F): These bakes are usually used to change amorphous structure to crystalline structure in both medium and high phosphorus electroless nickel plating. Because of the structure change of the deposit, hardness increases and corrosion resistance is reduced.
2) Low Temperature Postplate Bakes (< 500F): These bakes are often used to promote adhesion especially on aluminum alloys and relieve hydrogen embrittlement in ferrous alloys. There is little change in deposit properties with these lower temperature bakes.
3) Preplate Bakes: Many specifications call for preplate bakes or shot peening of higher stress ferrous alloys to promote adhesion and reduce the risk of hydrogen embrittlement. Preplate bakes don’t affect the properties of subsequent ENP right away
Among our many types of metal plating services, Aerospace Metals provides controlled heat treatment of EN designed to reduce or outright remove cosmetic changes in the appearance of EN deposits (like darkening) during the baking process. Sealers can also be added to improve corrosion protection performance of high-temperature baked electroless nickel deposits in order to enhance overall corrosion protection. For more information about solutions and troubleshooting challenging ENP applications, contact Aerospace Metals or check this article out at https://tinyurl.com/ynuehdsz.
Aerospace Metals offers full-service electroless nickel plating for most shape or sized component. For more information about ENP or any metal plating applications you may have, please contact me at Aerospace Metals at (800) 398-0790 for a quick and free estimate.