I see that Fernox are using the old screw test in water nonsense to try and fool installers into thinking they need chemicals in heating systems, I saw this on the Installer Magazine website http://www.installeronline.co.uk/see-shocking-affect-corrosion-can-heating-systems/ well lets just put some facts to this.
Such tests are nice things and produce results that are easy to control, and are impressive to people that do not know anything about corrosion parameters.
The screw in the inhibitor shows no sign of corrosion because the inhibitor either reduces the oxygen, or creates a film on the steel surface, so that no corrosion can take place.
The screw in pure water clearly shows signs of corrosion, but this is normal and is just the metal forming its natural protective stable layer, the question is why is the process continuing as the screw is now passivated, How much oxygen does the water still contain? What is the electrical conductivity of the water? What is the pH of the water.
This screw test has NOTHING to do with the operation of an actual heating system, in these screw-jar systems you have a large volume of water and only a little steel, whereas in a heating system this would be the opposite, a little water and lots of steel. In the jar test there is no temperature rise and thus you still have carbon dioxide present in the water, which will keep the pH low. The passivation of steel in natural water very much depends on the pH – this needs to be around 8.5 for the passivation process to be started and this pH would be achieved through temperature rise and the de-aeration of the system, this will not happen in the jar test and those showing these tests don’t want it to.
This jar test is just a theoretical test that cannot be compared to a real heating system, as the conditions are totally different.