Plate heat exchangers are widely used for heating and cooling activities in domestic and industrial applications. Eight plate heat exchangers of different construction and from various manufacturers were investigated, since all of them were leaking. The plates of all these components were made of stainless steel (mainly Cr17Ni12Mo2 or similar) and they were brazed with pure copper.
After cutting and metallographic preparation, the samples were investigated by light optical microscopy (LOM) and scanning electron microscopy (SEM) to measure the depth of copper penetration into the plates (liquid metal embrittlement). For seven heat exchangers the penetration depth was between 25 and 80 µm, but for one the depth was about 150 µm. At the steel-copper interfaces an additional phase was observed, which was identified as chromium-rich ferrite by electron beam micro analysis (EBMA).
To get information about the sensitivity against corrosion electrochemical tests were performed in two electrolytes (HCO3- + SO42- and HCO3- + SO42- + Cl-). Relevant distinctions, explaining the failing of the heat exchangers, were not observed by these tests. It can be summarized that in the electrolyte containing only SO42- and no Cl-, mainly the copper was attacked. In case of the electrolyte containing additionally Cl- besides copper also steel and in particular the interface zone influenced by the liquid metal embrittlement corroded.