Notwithstanding the effects of erosion due to wet steam acceleration through bends in the pipework, clean high-quality steam is not normally chemically corrosive in its gaseous state. But when steam is contaminated with impurities it can present corrosion and deposit risk to turbines, filters and steam lines when the steam or the impurities condense.
Corrosion from boiler water carryover
Steam is often contaminated with small quantities of boiler water carryover, which may contain dissolved sodium hydroxide, chloride, sulphate and silicate. With a pressure or temperature drop in the steam, the contaminants will lose their solubility and contaminate into the steam and condensate systems, which can cause stress corrosion cracking.
Sodium hydroxide carried over from alkaline boiler water will show a characteristic black magnetite deposit on steel surfaces, which can foul steam filters, accumulate in trap seats and generally drive black deposits around the steam condensate system. In the case of chloride, this will cause pitting corrosion in the presence of even a small amount of oxygen.
Corrosion from gases in the steam
Steam may also contain gas contaminants such as oxygen, ammonia, carbon dioxide and volatile silica gas. In addition, any gases that have not been eliminated from feedwater by pretreatment, or eliminated from the boiler by the correct chemical water treatment, can carry over with the steam and attack the after-boiler pipework. If sufficient oxygen gas is present in the system, either from after-boiler leaks, or from inadequate chemical oxygen removal in the boiler, pitting will occur in the steam lines even in the absence of chlorides.
Carbon dioxide gas carried in the steam from the boiler water can react with condensate to produce carbonic acid, although it is more common to see this take place in condensate lines rather that steam lines. Excess ammonia in the steam can attack copper based alloys.
How corrosive is your steam and condensate system?
If you suspect premature failure in your steam handling equipment, it is important to ask your water treatment supplier to test the chemistry of the feedwater, boiler water and condensate system to look for the signs of contaminants contributing to equipment failure. Not all chemical treatment programs may be optimised or compatible with the metallurgy, or operational demand of your particular steam system.