Corrosion is always a result of oxygen ingress.
Oxygen can enter closed systems in several ways.
Our Risycards 'RICA' explain the possible causes for this.
RICA 01 - Automatic air vents
An automatic air vent that suddenly becomes an ‘aerator’ turns immediately into one of the biggest causes of corrosion !
Although almost everyone thinks that automatic air vents can prevent corrosion (which is hardly the case*), in practice it turns out that they even allow air (and therefore oxygen) to enter the installation.
The real CAUSE of the problem lies, of course, in the failing pressure maintenance. This makes it the ‘Achilles’ heel for the life span of the heating installation.
RICA 02 - The accuracy of a pressure gauge and the green zone
A correct reading pressure gauge is very important and often a sore point in practice. Inaccurate pressure gauges, without green zone, or not adjustable, with reading errors provide poor information and cause misunderstandings. The result: troublesome and expensive consequences due to oxygen entry and thus corrosion sludge formation.
RICA 03 - Neutral point
Sometimes installations have a tendency to suck in air, again and again, while in theory this should not be possible, and apparently everything is in order (see our Risycards and Risybasics).
This is often caused by an incorrectly positioned neutral point, causing the circulation pump to create negative pressure, with the expensive and troublesome consequences of unwanted oxygen entry and thus corrosion (sludge formation).
RICA 04 - Failed Air Inlet Barrier in Pressure Step or Vacuum Degassing
Pressure step or vacuum degassing becomes a massive cause of corrosion if the air inlet barrier fails. This is a non-return valve that must prevent air (and therefore also oxygen) from entering the installation. Depending on whether it concerns vacuum degassing or atmospheric pressure step degassing in combination expansion systems, the amount of oxygen entering can differ greatly.
RICA 05 - Constant pressure expansion systems and the danger of oxygen permeable bladders
The constant pressure expansion system is a risk component due to the danger of an oxygen permeable bladder.
The bladder in the expansion vessel separates the oxygen-depleted system water (“dead water”) from the oxygen in the compressed air cushion (compressor system) or atmospheric air (pump system). This barrier is of the utmost importance to prevent oxygen entering the system and thus causing corrosion. Most common are butyl rubber (IIR) bladders which have the best resistance to permeation of all commercial rubbers. EPDM is also sometimes used, but has an oxygen permeability approx. 17x higher than that of butyl.
RICA 06 - Breakdown of indirect potable hot water heater
In the event of a leak of the heat exchanger that heats the hot water (PWH) oxygen-rich potable water can enter the heating circuit. This problem may go unnoticed for a long time, with catastrophic consequences in terms of corrosion and possible scale formation.
RICA 07 - Oxygen diffusion through plastics
Most plastics and rubbers are waterproof, but not gastight. Despite the fact that the installation is pressurised relative to the atmosphere, oxygen can still enter due to the difference in partial pressure.
RICA 08 - Explanation constant pressure pressurisation - content indication
The constant pressure expansion system is a risk component due to the frequent misinterpretation of the system pressure on the pressure gauge.
RICA 09 - Cl-open systems
Although these systems are marketed as closed systems (which they apparently are), they still cause serious oxygen entry, often resulting in massive corrosion.
In RICA 05 the importance of an oxygen diffusion tight bladder is explained, RICA 09 looks at alternative systems that do not use a bladder.
RICA 10 - Pre-pressure
The incorrect gas fill pressure (pre-pressure) is often the cause of corrosion damage, especially in connection with automatic air vents (see RICA 01).
Too high a pre-pressure is just as wrong as too low, and even a correct pre-pressure can quickly become too low due to pre-pressure loss.
The loss of pre-pressure is often compensated for by unnecessarily adding water instead of correcting the gas fill pressure. In practice, it is by far the most common cause of corrosion damage.