Monday, August 26, 2013

DIY active anti corrosion system

I'm part of Illutron. We have our main workshop at a large steel barge and we are always worried about corrosion.
Since we could not afford to have the barge sand blasted and painted we looked into other options.
Cathodic protection is a process where you actively impress an electric current between a ships hull and some anode. The replaceable anode is then corroded in place of the ship and everybody is happy.
The Anode may be any large clump of metal submerged next to the ship, e.g. an old engine block. We use 3 meters of rail track.
A variable voltage between 0 and 1.5V and a current between 0 and 10A is required.
For this I designed a simple power supply circuit:
Mains is first converted to 12V using an efficient industrial switch mode power supply. A low frequency Buck converter (low frequency=low switching loss) then lower the voltage further under microcontroller control.
Low frequency switching require a rather large inductor. The transformer like object in the picture below is a 55mH(1) inductor (schematic says 100, not critical) for the Buck converter. If you want to build this you may use the primary of an old microwave transformer (remove secondary!!).
( 1: used to say micro-Henry it need to say mili-Henry. Thanks Fred )

The microcontroller measures the output current and voltage and displays them on a standard 2x16 chr. LCD. I used an Arduino and connected the LCD according to the Arduino LCD example.
I use the somewhat exotic BTN7970 as H-bridge driver for the Buck converter. This offer very low switching losses but could be replaced by your favorite choice of H-bridge device.
The Arduino code implements the following functions:
By pressing the switches you can adjust the voltage in small steps. Press both switches to lock the keys from unintended access, press both again to unlock.
Up-time is displayed and a clearable message appears if the unit have been power-cycled.

Does it work? 


These two mild steel plates was submerged next to the barge. The left is isolated from the hull and the right is electrically connected to it.
This picture is taken after two days.

If you attempt to use this system on your own ship/structure you need to know what you are doing. Reversing the polarity result in the ship being corroded super-fast and running too much current result in calcium build up on the hull potentially stripping off existing paint.
You need to measure the corrosion potential and adjust the voltage accordingly.

You may use the following method if you are careful:
Connect a piece of blank steel to the hull with a beefy bable. Slowly increase the voltage over several days until the steel stays blank. Clean the steel between inspections with steel wool.
Good luck.