Given the highly corrosive nature of LOx and the extremely low temperature, what materials are usually used for the LOx pumps?
Given these limitations, what materials could act as candidates for the impeller and housing in the pumps pumping LOx?
In the Space Shuttle Main Engine a nickel "superalloy" called INCONEL 718 was used.
High-Pressure Oxidizer Turbopump - The function of the high-pressure oxidizer turbopump (HPOTP) is to provide a high-pressure, high-volume flow of liquid oxygen to the main injector arid to the preburner injectors sufficient to ensure positive injection of oxidizer at all thrust levels (Figure 8). The HPOTP is a centrifugal pump consisting of a double-entry main impeller and a smaller, single-entry boost impeller on a common drive shaft and develops 16.4 x 7 10 W at 29,000 rpm. The low-pressure oxidizer pump delivers liquid oxygen to the HPOTP main impeller at about 2.9 MPa. The main impeller increases this pressure to almost 35 MPa and most of this flow is directed to the MCC injector. A small portion of this liquid oxygen flow is diverted to the boost impeller which increases the pressure to approximately 55 MPa to feed the prebumers. The main impeller is machined from an alloy 718 closed-die forging and the boost impeller is an alloy 718 precision investment casting. The pump housing, also alloy 718, is a complex welded assembly composed of castings, forgings, and formed sheet. All components are in the solution-treated and aged condition subsequent to welding. Alloy 718 was selected for its high strength, compatibility with liquid oxygen, and good toughness at the operating temperature of -179°C.
Liquid oxygen is not corrosive in the traditional sense. There is no electrochemical attack.
It can make many materials much easier to ignite. For a fire or explosion to occur, all three parts of the fire triangle must be present. Oxygen is one, fuel is another (metals, plastics, actual propellants) and an ignition source is the third.
An ignition source can be the traditional source, like spark, direct heat, or from particle impact (debris hitting a piece of metal or plastic and imparting enough energy to cause ignition), adiabatic compression (oxygen itself rapidly slowed by closing a valve, turning a corner), or others.
Metals and plastics all will burn to a certain extent depending on pressure, temperature, and chemical composition. Materials can be perfectly acceptable under some conditions and completely unusable under others. The higher the pressure/ temperature the more likely that an ignition event can occur.
In general, materials fabricated from copper are the best but tend to be lower in strength and not tolerant of high temperature. These include brass, copper, monel, toughmet. However, even these will burn in certain conditions, but due to the high thermal conductivity can cool rapidly. They also do not become brittle in cold temperatures. Many of these will not burn until pressures are near 10000 psi.
Nickel based alloys are the next likely material used, if off the shelf materials are being used. They are high in strength but can readily burn above 800 or so psi. Sometimes coatings are used to help with ignition, but Inconel 718, Inconel 625 and others can ignite at pressures less than 1000 psi.
Carbon steel is never used, as it's easily ignited and becomes brittle at these temperatures.
Materials like titanium can ignite even at room temperature and aren't used.
Material like raw aluminum can also ignite at low pressures, 25 psi. But add anodization and aluminum is often used on the low pressure suction side of the pump.
On the high pressure discharge side aluminum is not typically used as risk is too great of an issue.
Many rocket companies develop custom alloys to deal with issues.
Like all things in engineering, there is not just one material that will work, everything is configuration dependent.
