Final answer:
The true statement is that all brazing alloys containing phosphorous do not require flux when used on copper because phosphorous itself acts as a flux. Other statements are incorrect, as soldering alloys have a lower flow temperature than brazing alloys, and copper-containing solders may still require flux. option 4 is correct
Step-by-step explanation:
The correct statement is:4) All brazing alloys containing phosphorous do not require flux.
This is because brazing alloys that contain phosphorous are self-fluxing when used on copper. Phosphorous acts as a flux, which is a substance used during the brazing process to prevent oxidation, to allow the filler metal to flow more easily, and to create a stronger bond. However, when brazing certain metals such as steel or nickel alloys, a flux is often still required even if phosphorous is present in the brazing alloy.
Contrary to option 2, soldering alloys generally have a lower flow temperature than brazing alloys, which allows them to melt and flow at lower temperatures without melting the work pieces. Soldering with a copper-containing alloy may still require flux, depending on the type of solder and the metals being joined, which makes statement 1 incorrect. Lastly, statement 3 is incorrect because phosphorous-containing brazing alloys are typically self-fluxing on copper base metals and do not require additional flux.