Wednesday, February 25, 2009

Stainless Steel

Brass Valve Material FAQ

Piston valves in modern brass instruments are made primarily of three materials, Monel metal with brass ports (un-plated), Nickel Plated Nickel Silver with brass ports (plated), or Stainless Steel with brass ports (un-plated).

What makes them good
Monel has 65% nickel in it. Nickel plate has 100% nickel on it. Stainless Steel has 18% chrome and 8% nickel in it.

Arguments for each can be made depending on criteria used. It is important to identify the criteria before making a claim as to which material is superior.

A claim is frequently made for Monel as superior to Nickel Plate claiming that Monel does not wear as much as Nickel Plate. All valves wear. If they did not they could not be honed and lapped to the fine tolerances needed. In truth, Monel does not “show” wear like Nickel Plate because there is no plating to wear off exposing the areas of wear. Typically when lapping a valve, lapping compound imbeds in the softer material (the brass casing) and cuts the harder material, the piston. Dirt, it could be argued, can act as a lapping compound. This would imply that whether Monel, Nickel Plate or Stainless Steel, valve wear is primarily on the piston itself. That does not mean there cannot be wear on the casings too. Nonetheless, what many people call wear is, in fact, simply visible wear as seen only on plated valves when the plating wears off. This is not the whole story. The primary factors in wear are cleanliness and lubrication.

Coefficient of Friction and Oil Retention Whether sliding friction or static friction, the ‘slipperiness’ of the piston is often cited as a criteria for superior valves. A case can be made that Nickel Plate is best when this is the main issue. However, the slipperiness of the material also implicates it as worst for oil retention. In fact chrome plated pistons (not currently popular) are even slipperier but must have pronounced cross hatching from lapping to retain oil. In practice, nickel plated valves are frequently not hand-lapped to the casing making them less tight and canceling any benefit from reduced friction. Additionally, Nickel Plated valves of necessity have nickel plating inside the ports which alters the acoustical properties. The arguments change in the presence of different lubricants.

Corrosion resistance and susceptibility to electrolytic reaction
Resistance to corrosion and susceptibility to electrolytic reaction with the valve casing can be a significant factor for players with highly acidic or saline body chemistry. Greenish or brown build-up is not uncommon on Monel valves. This is due to an electrolytic reaction between the piston and the casing. The discoloration is actually brass from the casing itself as it leaches on to the piston. Nickel plated valves and stainless steel valves are less susceptible to this reaction.

Other Considerations
Thermal expansion is not frequently considered when addressing the merits of valve material. Whether a piston expands at the same rate as the casing can be a factor especially in cold weather marching bands.

Although tensile strength is sometimes cited as a consideration, unless poking a nail into the piston or rough use when removed from the casing the instrument is contemplated, this is usually not an issue.

Comparing the options
While the chart below cannot be considered a formula for comparison, it does rank the materials on the specific criteria. E.g. If your criteria is coefficient of dry sliding friction, Nickel Plate is best. If your criteria are visible wear or flaking plating, Monel and Stainless are preferred over Nickel Plate. If preserving the acoustics of the continuous brass tube are the criteria, Monel and Stainless are preferable to any plating. Your needs may emphasize one criteria more heavily than another. If considering trouble-free performance, stainless steel has the edge. If taking all factors into consideration, Stainless Steel will serve best in most situations.