As a developer in a company that builds industrial controllers I want to point out some differences between Arduino and "real" industrial controllers:
The arduino hardware consists more or less of a "naked" microcontroller with 3V3 or 5V GPIOs that are directly forwarded to the connectors for user experiments. On industrial controllers you can also find the same or similar microcontrollers but their GPIOs are never directly accessable. The GPIOs are always equipped with level conversation (like 24V) and with several protection circuits. For Inputs you normally have overvoltage protection, filters and hysteresis circuits against EMC disturbances like surge or burst. For outputs you have current amplication to drive currents of several Ampere and often also protection against overload and short circuit. All this helps the microcontroller to survive in an industrial environment.
Another difference is that industrial controllers have to be tested against several industry standards (IEC, IEEE) for EMC, electrical safety, environmental conditions, mechanical shock and vibrations and so on. A naked Arduino definitly wouldn't pass these tests.
So yes, in theory one could build an industrial controller USING an Arduino, provided that it is equipped with a "shield" as well as a mechanical enclosure that are appropriate to meet the above requirements, and to pass the industrial standard testing.
Simple industrial controllers (as already suggested) like a Siemens Logo start at € 100 - €150.
Since you mentioned industry standards and "Shields" - how does the nowadays very common "stacking using headers" connections fair as "industrial grade" connections?
Granted the whole hardware setup would not be directly exposed to moisture etc. but how about vibration, temperature changes etc.
Now little OT.
I have briefly worked with "down hole" AKA oil exploration hardware and the common environmental setup / protection was encapsulating the silly narrow ( 2 inches tops ) and very long ( I have seen 2 feet long ) PCB in practically irreparable( good for repeat business HA HA ) goop.
Most electrical failures were with "bypass capacitors". Unfortunately after mechanically removing the goop the caps grade ( mostly operating voltage , sometime temperature code / color ) were unreadable.
Needless to say - all components were soldered to the PCB,no CPU hardware / software of any kind was used.
But things probably changed after 10 years.