Stainless steel comes in a variety of alloys with diverse properties that are useful in manufacturing, engineering and electrical applications. Although stainless steel may be formed and joined in a variety of special ways, soldering stainless steel presents adhesion challenges. There are specialty fluxes and solder types, but, for components that are not required to undergo heavy stress loads, could there be a superior methodology?
As an electroplater and electroformer for 23 years for the National Radio Astronomy Observatory, I had occasion to work with stainless steel fairly often. One of the jobs I was assigned was the preparation of stainless steel jacketed rigid coaxial cable to be soldered into connectors. I was able to develop a methodology that worked for most of our applications.
Surface Preparation for Electroplating
Having worked with a variety of metals, I became well acquainted with the importance of surface preparation. For instance, aluminum metal is really quite reactive. It would not last long in nature, except for one fact that guarantees its existence. Aluminum forms a kind of skin – an oxide coating, chemical formula Al2O3. This coating forms practically instantaneously on freshly exposed aluminum surfaces. Once it forms, it stops air from further attacking the aluminum underneath it.
If one desires to coat the aluminum with another metal, one must not only remove this oxide coating but cover it extremely rapidly before the oxide layer can reform. I wondered if something similar might occur in the case of steel. Certainly if an oxide layer of something similar formed on steel, there were ways of dealing with it. I decided to try what I called “hydrogen plating.”
Converting Cations to Atoms
Most electroplating involves converting cations to atoms. What is a cation? A cation is a positively charged atom-an atom missing one or more electrons. We discuss copper in this article. Cu+2 is a cation-a copper atom missing two electrons, readily obtained by dissolving copper sulfate (CuSO4) in water,
CuSO4 (upon dissolving in water) → Cu+2 + SO4-2
Electroplating involves an electric current. The negative electrode (the piece to be plated) is rich in electrons and draws the copper cation to it, where it plates out as a layer of copper.
Copper plating a piece of clean stainless steel does occur in the deposition of a lovely sheet of copper on the steel. The problem is, it easily peels off with a piece of cellophane tape. It conforms to the surface of the steel, but does not adhere to it. Could this failure to adhere be due to an oxide or some other surface coating? I decided to reduce any oxide present by hydrogen “plating.” To accomplish this, I chose1 to use 30 percent sulfuric acid solution, applying 125 amps-per-square-foot of surface area for a period of not less than four minutes.2 What, in this case, is the cation that plates the surface? It is the H+ cation to yield a single atom of hydrogen, also called monatomic hydrogen, H.
H2SO4 (upon dissolving in water) → 2 H+ + SO4-2
An added benefit to using sulfuric acid is chemical compatibility. Both the hydrogen plating and the copper plating use the sulfate negatively charged ion, or “anion.” Taking a plated piece out of the sulfuric acid and putting it without rinsing into the copper sulfate solution would not introduce a contaminant. Especially is this so because copper plating baths also contain some sulfuric acid, by design.
Hydrogen cations are extremely small-in fact, they are naked protons. Not only can they form hydrogen atoms on the surface of the metal but any steel oxide can be reduced to water molecules and oxide-free metal, improving the copper plating that follows. In fact, this process works. It is a proven methodology. No theory is necessary to carry this procedure out. Yet, the scientific mind craves an understanding of what is going on. For this reason, I have tried to evaluate what is going on. For years, this hydrogen-plating concept is how I would explain the process. Yet, now, I think there is an alternative possibility that makes sense.
Since I have retired and read more on the subject, I now have a different perspective. Stainless steel, if treated with hydrogen, develops some measure of “hydrogen embrittlement.” This is a condition that can severely weaken the metal and has led industrially to fatigued parts that have broken under stress-load conditions, resulting in loss of life. Hydrogen embrittlement is a very real phenomenon. What causes hydrogen embrittlement?
It is a very complex issue, and all the answers are not yet in. Hydrogen ions, atoms, or molecules are very small and can readily enter between the atoms in the steel matrix. That is, they can penetrate the surface of stainless steel. They can then coalesce or expand in some fashion to produce voids and tears and other weaknesses within the bulk of the steel. Sandia National Laboratory offers a very instructive article with this most interesting microscopic image of embrittled stainless steel.
Metal Finishing Magazine’s online website, in a discussion of hydrogen embrittlement, quotes a reader, Bill Boatright, who indicates the higher the current applied in electroplating a stainless steel piece, the greater is the danger of embrittlement.
Now here is the important thing: since the hydrogen-plated coaxial cable has received a high-current density exposure, there should be many such voids and cracks. Follow-up electroplating with copper may partly fill in some of these voids and cracks, providing anchors for the copper plate to adhere to the stainless. Hence, this may well be the mechanism by which the copper-plating methodology, above described, works.
1This methodology was actually refined gradually over a period of time, by trial and error.
2Other electroplating methods I used are described on my NRAO Electroplating and Electroforming Work Page. I am retired from the NRAO.
References and Resources:
Stanford National Accelerator Laboratory – Hydrogen Embrittlement of Steels
Worcester Polytechnic Institute – Hydrogen Embrittlement and Electroplating
Duro-Chrome – “Hydrogen Embrittlement: How Small Details Can Have Large Effects”
11th International Conference on Fracture – Hydrogen Embrittlement of Stainless Steels