Corrosion mechanisms and models for flue gas corrosion in aluminum heat exchangers

Aim

In condensing boilers for home appliances, but also increasingly for cascaded systems in commercial/industrial buildings, condensate is produced in the heat exchanger and flue gas ducts. This is done to maximise the thermal efficiency. This condensate has to be removed by a drain system, consisting of a condensate collector, siphon, and additional drainpipes. In the condensate collector the condensate from heat exchanger and flue gas duct flow together towards the siphon. In the siphon (or downstream of the siphon) sometimes also expansion water from domestic hot water tanks or other tap water sources are added to the effluent streams.

During tests there have been observations of heat exchangers and drain systems contaminated and/or clogged with corrosion products within a few months time. A clogged heat exchanger leads to malfunctioning of the appliance, which, of course, must be avoided.

Also, the demand for higher efficiencies and a more compact construction leads to heat exchangers with narrower gas channels that clog easily because of the corrosion and this is an urgent problem in the new boiler models that will reach the market in the following years.

In order to improve the operational performance of the heat exchanger and to be able to reach the target of a maintenance free system, reliable design rules and/or operating philosophies (modulating / non modulating) are needed for these kind of systems. For this purpose a better understanding of the underlying processes leading to contamination/clogging of drain systems should be obtained.

First results

With the purpose of validating and extending the experimental results that already are in Bosch TT’s possession, immersion corrosion tests have been done. The corrosion conditions are divided into two different categories, depending on the condensing operation of the boiler, namely mild conditions and harsh conditions. The microstructure of original and corroded sample surfaces was studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro-analysis (EPMA). The corrosion mechanisms are revealed: the corrosion tends to attack first at the intermetallic phase (IMP) which is located at the grain boundaries between the aluminum rich phase and the silicon flakes. Then, due to the degradation of the aluminum phase around the IMP, the latticed Si phase is exposed on the surface.

Ongoing Work

A test rig for simulating dew point corrosion has been constructed and established. Several tests were carried out in order to find suitable parameters for the control and smooth operation of the test rig. The microstructure of the corroded sample surfaces was studied using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) has also been used to examine the corrosion behaviour of AlSi12 alloy in diluted sulphuric acid solution because of its capability of characterizing different relaxation phenomena.