Recently we found rust on the solenoid valve tube. But the funny thing is we did not found any rust before. Does different batch of this stainless steel can give different properties? If yes, what can we do to check it?  

This is quite typical for 303 stainless steel tubes, which are a high sulphur free machining grade stainless. You may be having a sulphide problem and not a rusting problem at all. Sulphides will bloom to the surface when exposed to moisture and look very similar to rust. Try soaking the part in 10% NaOH (Sodium Hydroxide) overnight and see if it goes away. If it does go away or wipes off it is sulphides, not rust. 

Solenoid valves can get rusty and stop working, usually the signs are that the valve typically in the open position and fails to close, usually causing the solenoid coil to over heat and burn out, especially when used on AC 50/60Hz power supplies as the coil will stay in the higher inrush position. When you dismantle the solenoid valve you will see clear signs of rust build up on the armature assembly inside the core tube where the solenoid coil is mounted.

However, if you are using a quality solenoid valve with the correct grade of stainless steel internal parts you will find that the rust deposits will wipe away with a damp cloth leaving a shiny stainless steel surface without the signs of pitting and corrosion. 

In this case the rust has only accumulated here from the water passing through as it has been trapped due to the magnetic field generated by the solenoid coil. You will typically find various degrees of rust particles even in mains water supply lines depending on location and the age or type of supply pipeline. However in many cases the rust or metal particulates have either been generated from other water process equipment upstream or from metal debris left inside the pipework during installation process. Keep in mind that initial installation work pipelines have threads cut into them, metal dust from grinding, cutting and fabricating can remain inside the equipment if it has not been cleaned before being installed and ends up being flushed downstream and magnetically trapped by the solenoid valve. 

Helpful hint: Avoid pouring neat cleaning chemicals into new pipework systems even if water is added behind it later as this concentrated cleaning solution will cause corrosion downstream, especially as it can become trapped in dead end lines, crevices etc, which will remain stagnant for some time. 

If you have purchased a cheap solenoid valve, basically you get what you paid for and will probably find the internal parts are pitted and corroded. Best option is to replace the solenoid valve with a quality brand product such as a Shako solenoid valve that has rust free stainless steel internal parts.

You can prevent rust particles in the system by cleaning the process pipework and equipment before installing, installing quality equipment that does not contain ANY rust generating materials and by fitting a magnetic descale unit upstream.