Older yellow brass horns can develop what are known as rose spots or red spots or red rot. This is an area where the zinc in the brass metal matrix is selectively leached out leaving a porous area of reddish appearing copper. Its called dezincification. Sometimes a white powdery substance can be seen. This is zinc oxide. Brass that is more than 15% zinc is particularly susceptible to dezincification. Special alloys of brass can be made that are dezincification resistant (DZR brass). The dezincification process is driven by water that is high in dissolved carbon dioxide, oxygen and sulfur.
There are generally two genres of dezincification; plug-type and uniform-layer. Uniform-layer dezincification results in an even thinning of the material, while plug-type dezincification results in discrete areas where the corrosion process is most notable. It can cause holes in the material.
In the parts of the horn consisting of tubing, the dezincification process can cause structural weakness and lead to leaks. On the surface of the valves, dezincification can result in the accumulation of gritty zinc oxide and cause frozen valves that require disassembly and cleaning. Then one hopes that it won’t happen again after the valves are reassembled. a
A penetrating plug-type dezincification in the valve rotor or casing of a horn that results in a connection between the B-flat and the F side of the horn could result in an even worse problem. I’ve heard of a Conn 6D bought on EBay that could not be played on the F side but only the B-flat side. A strategically placed hole in the thumb valve rotor or a groove in the casing at the 6:00 o’clock position could do that by connecting the B-flat and F sides when the horn is standing in F but once the valve is switched to B-flat the F side of the horn is completely out of the loop and so the horn works. The point is that corrosion and dezincification can ruin a horn. This problem can be avoided by making the valve rotor and bearings and valve casing out of material that is not prone to dezincification or other forms of brass corrosion.
If you look at a table of galvanic corrosion you will find that nickel silver is in the same territory as stainless steel, whereas the yellow and red brasses are much more prone to corrosion. I can report that in all the time I have had a Briz horn there has never been corrosion between the valve and the valve casing.