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Electroless Nickel Plating as a Brazing Filler Metal

A number of questions have come up in recent months about the use of electroless nickel plating as a brazing filler metal (BFM). To begin with, yes, electroless nickel (EN) plating can be an effective BFM, when properly applied. The eutectic nickel-phosphorus alloy composition (89Ni-11P) is already available commercially as a separately purchased BFM in powder or paste form from different manufacturers, and is listed in the American Welding Society (AWS) Specification A5.8 with a “BNi-6” designation.

Please note that the process of “electroless” nickel plating is quite different from “electrolytic” nickel plating, and their end-products are also very different. Electroless nickel will deposit a nickel-alloy (either nickel-phosphorus, or in some rare cases, nickel-boron) onto a surface by chemical means (no electricity being used), which (because it is an alloy) can start to melt at temps as low as 1616°F (880°C), whereas electrolytic nickel plating uses electricity in a chemical bath to deposit a layer of pure nickel onto a substrate, which will not start to melt until 2651°F (1455°C). It is VERY important that end-users of nickel-plating thoroughly understand this difference, or brazing problems can (and have) resulted!

So, according to the Nickel Development Institute (and as I just mentioned above), electroless nickel (EN) plating is a process that does not require the use of any electricity to deposit a nickel alloy from aqueous solution onto a substrate, whereas, by comparison, electrolytic nickel plating requires an external source of direct current to reduce nickel ions to nickel metal on the substrate in an electrolyte bath. Once again, electroless nickel plating accomplishes this reduction of nickel ions to nickel metal by chemical processes, without the use of any electricity.

The most common chemical reducing agent used in EN-plating processes is sodium-hypophosphite (probably used in more then 99% of all electroless nickel plating processes). During this chemical plating process, the hypophosphite is converted to just regular sodium-phosphite with the subsequent depositing of nickel in accordance with the simple chemical reaction:

formula

However, the actual chemical reaction is apparently much more complex than just this simple equation, because the Ni doesn’t deposit as pure nickel, but as an alloy of nickel and phosphorus. Thus, there will, in fact, be a significant percentage of phosphorus (abbreviated to just “phos” in a number of places in this article) in the composition of the nickel layer that is deposited by this EN process, the phos content ranging from about 2%-to-13%.

The industry normally identifies electroless nickel coatings according to their phos content, for example:

Low phosphorus ------ 2 - 5% P. Medium phosphorus -- 6 - 9% P. High Phosphorus ------- 10 - 13% P.

Be sure you know what you are getting from your particular plating supplier. Melting Point

As mentioned earlier, pure nickel has a melting point of 2651°F (1455°C) but the addition of phos to the nickel has a very significant effect on its melting point, as shown in Figure 1.

Fig 1 lg
Fig. 1 -- Effect of Phos content on melting temperature of Ni-P alloy (Diagram courtesy of the Nickel Development Institute)

The final melting point curve declines almost linearly from 2651°F (1455°C) to 1616F (880°C) for an alloy containing 11% phosphorus. This is the lowest melting point (eutectic) for the nickel/phosphorus system and some melting will occur at this temperature, regardless of the phos content, provided it is greater than 0.2%.

This is very important to know, since it can affect the flowability of the Ni-P plated layer when it is re-heated and melted during your brazing process. Having said that, please be aware, however, that melting of the EN-plated Ni-P layer will always start at approximately 1616°F (880°C), and will either flow freely and quickly for EN layers with high phos contents, or much more sluggishly, and over a much wider temperature range, for low phos contents. The reasoning behind this is discussed in much greater detail in my earlier article regarding “liquation”, and the reader is referred to that article for further clarification on this topic. To aid the reader in that further study, if interested, the actual full phase diagram for the Ni-P system will be needed, and so, is reprinted here in Fig. 2.

Fig 2n lg
Fig. 2 Ni-phosphorus equilibrium phase diagram, reprinted from “Der aufbau der zweitofflegierungen” by M. Hansen, Edwards Bros., Ann Arbor, MI, 1943. “Schmelze” means “liquid”. The bottom axis shows the increasing weight-percent of phos, being eutectic at 11 wt-%. The horizontal line at 880C is the solidus of the Ni-P system, everything below that temp being solid in this diagram (up to 15 wt-% of P)

Thickness Uniformity

Another significant difference between electroless nickel deposits and those deposited via electrolytic nickel plating is the ability of EN-plating to produce deposits with a very high degree of thickness uniformity, as shown in Fig. 3. Note that this uniformity extends into pockets, etc., which can be very important for trying to place a uniform coating of a Ni-P brazing filler metal (BFM) layer. The “throwing power” in an electrolytic bath is such that sharp corners tend to build up much faster than surfaces down inside pockets, or deep curved surfaces, etc.

Fig 3 lg
Fig. 3 Note the uniformity of the electroless coating as compared with that of the electrolytic-nickel deposit. ( Drawing courtesy of the Nickel Development Institute)

Surface Preparation

As with any surface that is to be brazed, the metal surface must be clean and free of oils, dirts, grease, lubricants, and oxides, prior to any attempt at EN-plating.

Degrease first. A good alkaline cleaning solution should effectively remove any oils or lubricants. There are many effective alkaline cleaners out there, and your plating company should be able to guide you with this, based on the types of oils/lubricants on the surface, and the type of base-metal being plated.

Oxide removal next. Once the surfaces have been thoroughly degreased, then oxide removal can be done, if it is required, since any surface oxides are underneath the oil/lubricant layer. Oxides can be removed by a number of different methods, including grit blasting with a metallic grit, or chem-etching, acid-pickling, etc.

Preplate with a “nickel-strike”? Many people may not be aware that on some base metals, such as many stainless steels, it may be necessary to put a very thin layer of pure-nickel onto the surface of the metal first, so that the EN-plating will effectively bond to the base metal. This very thin layer of pure nickel, such as that known as a “Woods nickel strike” will effectively place a very thin layer of nickel, about 0.0001”-to-0.0002” (0.0025mm-to-0.005mm) on the metal. Your nickel-plating company should be able to guide you with more details about this.

EN-plating thickness. As with any brazing filler metal (BFM), you only need enough of a plated layer to provide the quantity of molten BFM needed to complete the braze. This is usually about 0.001”-to-0.002” (0.025mm-to-0.05mm) or so. If you plate a lot thicker than this, you may encounter excess BFM flow, base-metal erosion, etc., just as you would encounter with standard externally applied BFMs.

Brazing Results

Shown in Fig. 4, 5, and 6 are examples of brazed joints resulting from “preplacing” hi-phos (about 11% P) onto 316-stainless steel components, and then brazing them in pure dry hydrogen at 1800°F (980°C), thus representing a brazing temp approximately 175°F (100°C) above the eutectic temp for the Ni-P system. Although these photos come from a 1957 report by the Oakridge National Laboratory, they are, in my opinion, excellent representations of how the EN-plating process works for brazing applications, and the results presented are still very accurate, and very helpful for readers today!

In Fig. 4 you can clearly see the alloying that occurred between the plated-BFM and the stainless, as evidenced by the “fingers” of primary base-metal crystals extending into the fillet area from the base metal surfaces.

Fig 4 lg
Fig. 4. Cross-section of an EN-plated 316-stainless steel joint. The joint was brazed in pure dry hydrogen at 1800°F (980°C) for 10-minutes at brazing temp. (Photo courtesy of Oak Ridge National Laboratory’s ORNL-2243 report: “Electroless-Plated Brazing Alloys”, 1957)

Can I plate a ternary Ni-Cr-P alloy via an EN-process?

I do not believe that this can be done. I have never heard of someone doing this, and when I checked with a number of plating companies, they had not heard of anyone doing this successfully either (for reference, speak to Gary, one of the experts at Ultra Plating in GreenBay, Wisconsin. In his 35-years of plating experience, he has not encountered this kind of ternary plating, and gave a number of technical reasons why it would be difficult or impossible to achieve with today’s technology).

So, instead, in order to achieve a ternary Ni-Cr-P layer of BFM via plating, it is necessary to do a double plating job, just as they did back in 1957. Firstly, as shown in Fig. 5, plate the base metal with a standard Ni-P layer via an EN-plating process, followed by applying a very thin layer of electrolytic-Cr. And then, as shown in Fig. 6, at brazing temp the Cr will alloy into the Ni-P BFM and the new ternary-composition (Ni-Cr-P) will form and flow into the joint, providing a more corrosion resistant BFM in the process. Note the difference in the aggressive behavior of the ternary composition in Fig. 6, brazed at the same time and temp as just the Ni-P shown in Fig. 4.

Fig 5 lg
Fig. 5 Approximately 0.0002” (0.005mm) layer of pure chromium was electrolytically plated on top of a 0.0001” (0.0025mm) EN-plated layer of Ni-P in order to braze two 316-stainless steel components together for greater corrosion resistance. (Photo courtesy of Oak Ridge National Laboratory, op.cit)  
Fig 6 lg
Fig. 6. Microstructure of this Ni-Cr-P brazed joint, after heating in pure dry hydrogen at 1800F for 10-minutes at temp. (Photo courtesy of Oak Ridge National Laboratory, op.cit)

CAUTION re hardness of phos-containing BFM.

Caution is recommended whenever trying to braze any iron-based metal using a phos-containing BFM. Phosphorus loves to alloy with iron to form iron-phosphide phases in the brazed joint, which are, unfortunately, very hard, with little (if any) ductility. Consequently, as a number of industries have discovered over the years when using either copper-phos BFMs or nickel-phos BFMs when brazing any iron-based metals, although the joints may look beautiful, and exhibit excellent BFM flow and gap-filling capability, the resulting microstructure in the joint may possibly crack quite readily under any type of physical or thermal stress, resulting in joint failure or significant leakers. Therefore, please be aware of this before using any of these phos-bearing BFMs on any iron-bearing base metals.

CONCLUSIONS

Yes, people can use an electroless-nickel (EN) plating process to put a layer of a nickel-phosphorus (Ni-P) alloy onto a substrate for subsequent brazing. You will need to work with a good, competent electroless-nickel plating company to determine if an electrolytic nickel strike process will be needed after cleaning the metal in order in insure that the subsequent EN-plating will adhere properly for your particular brazing need, and to also control the actual percentage of phos that will be contained in the EN-plated layer.

Ternary-alloy plating via an electroless process is not currently viable, and would need to be done via a two-step process, first conducting a standard Ni-P process, followed (if needed) by an electrolytic Cr-plating process. This creation in-situ of a ternary Ni-Cr-P alloy is done to increase the corrosion resistance of the brazed joint.

Finally, due to the inherent high-hardness of any iron-phosphide phases that form in the brazed joint upon solidification of any phos-containing BFM on any iron-based metal, you will need to verify, in advance, if such phos-containing brazed joints will meet the service requirements of the joint in service, as far as thermal stresses, vibration, fatigue, etc., is concerned.


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