Carbon Steel:

Carbon steel is normal steel, it is widely available and inexpensive compared to other metals.  Steel is suited for use in a biodiesel processor but will corrode over time.  For example, a 55-gallon drum is a common material used in the construction of a biodiesel processor, under most circumstance will corrode to a point of replacement after about a year.  The steel corrodes due to air coming into contact with it's surface not because of the chemicals.  Steel plumbing is often used in biodiesel processors, it is the best plumbing choice that can be obtained by the normal hardware store.  Steel plumbing is often known as Black plumbing or Black steel plumbing.  This plumbing is very inexpensive and strong.  However black steel plumbing will corrode over time if air contacts it.  Therefore the exterior of the plumbing is the first to corrode because it is always in contact with air.  The outside of black plumbing can be painted to prevent oxidation, however the inside of plumbing and the inside of tanks can not because the paint will only erode as liquid quickly rushes over it.  Carbon steel is the best choice for a low budget processor.

Stainless steel:

Stainless steel is the best choice for processor construction as it will not corrode or stain, however stainless is the most expensive metal on this list.  Stainless is compatible with biodiesel and methoxide, but it is often too expensive for the homebrewer and is usually used in industrial applications.  Stainless plumbing is far superior to any other type of plumbing but it is about four times the cost of black steel and is not available in the normal hardware store, it must be ordered from an industrial equipment dealer.

Polyurethane:

Polyurethane is a plastic product that is chemical resistant but it is not heat resistant.  Poly tanks are the white variety that can be purchased at northern or tractor supply.  These tanks are often used on production biodiesel processors such as the fuelmeister, they come in a variety of wall thicknesses, sizes, and configurations.  Poly tanks are made to different configurations to accomplish different purposes, some are made to fit in the bed of a truck while some are cone bottomed.  The coned bottom variety are the most valuable to for the biodiesel processor as they provide an easy way to drain the byproduct glycerin after settling.  If you decide to use poly tanks be sure to pay attention to the maximum rated temperature because if the liquid the tank contains gets to hot then it can easily melt through the poly tank, always causing a hot mess and sometimes a fire.

Brass:

Often times the ball valves used on biodiesel processors are made of brass as it is inexpensive compared to other valves.  Brass is compatible with biodiesel and methoxide, and it will not corrode.  Brass is not as strong as steel and one must be careful when assembling plumbing because brass fittings are easily stripped.

PVC:

White PVC plumbing is not suitable for biodiesel as it is drain/waste/and vent plumbing, it is not pressure rated and has very thin walls and will melt when subject to the heat needed to process biodiesel.  The gray variety of PVC is suitable for biodiesel it is pressure rated and designed for use with chemicals thus it won't corrode.  Gray PVC will melt and it should be used in conditions where the liquid passing though it is not over 140 degrees F.  PVC tube is a clear flexible pipe which can be used in the processor where hard plumbing is not desired.  PVC tube will not stand up to very high heat and will break down over time must be replaced.  As the tube breaks down it will begin to sag and become weak.

Galvanized Steel:

Galvanized steel is not suited for biodiesel, as it's galvanized coating will corrode and affect the quality of the biodiesel.  Galvanized steel plumbing is well suited for water and it can operate under higher temperatures and hold pressure but it is not suitable biodiesel or methoxide.

Aluminum:

Aluminum is not suited for methoxide, as the caustic methoxide will corrode the aluminum.  Biodiesel does not react with aluminum but leftover catalyst and methanol in the fuel will making it not the ideal material for the application.  Aluminum is difficult to work with as it requires special welding abilities and it is quite expensive for the homebrewer.  Aluminum does not corrode when in contact with air or water.