Cathodic Protection

Obviously there will be online (some times) free courese- read literatures

But if you want a certificate it definitely won't be free so there.

Things with no cost often have no value.

There are plenty of free resources on the internet you can use for self-learning which you can also use to show prospective employers understanding of a topic, but if you really want a piece of paper to prove to an employer you have passed a course then you will most likely need to pay for the course (online or otherwise).

Yes there are some free courses on the internet (perhaps even on cathodic protection) that you can get a certificate, but the extra piece of paper is not really proving anything unless it is from a reputable source (which won't be free). Additionally you want to be able to actually learn something useful in the end.

My advice is self learning using free internet resources followed by a university or online short course (which you will need to pay for).

Free Online Courses with Certificates - Michael Gates