Prakt. Met. Sonderband 38 (2006) 349 
The samples are cold mounted and then prepared metallographically. 
After polishing the samples are first examined by light optical microscopy (LOM). 
By optical To characterise the microstructure of the samples various etching procedures (different 
ig. 2). The etchants and times) are used. 
brown with = Nital = 2 % HNO3 in methanol 
aken from = FeCls = 2.5 g FeCls, 15 ml HCl and 120 ml HO 
= V2A-pickle =1 ml HNOj3, 10 ml HCI, 10 ml H,O at 50°C (dilute aqua regia) 
etching time was about 10 seconds for etching the Cu (short) 
and 1 — 2 min for etching the stainless steel (long). 
For special effects also the etchings are used in various chronological orders. 
The samples are further characterised by scanning electron microscopy (SEM) and 
electron beam micro analysis (EBMA). 
3. SAMPLE CHARACTERIZATION AND DISCUSSION 
3.1 REGION OF A CORRODED BRAZE DOT 
The region of a corroded copper braze dot is investigated (Fig. 4). The cross section 
shows that there is intercrystalline corrosion in the steel plate on the corroded water side. 
The visible depth of the corrosion into the steel plate is about 50 um (Fig. 4b). 
ater = The corrosion products deposited on the steel plate and covering the braze dot are 
investigated by polarized light (Fig. 4c). Red particles (most likely CuO) are imbedded in a 
green-blue film (carbonatic deposit from the water). 
100 Hr 50 um 50 pr 
Fig. 4: Cross section through a corroded copper brazing-dot on stainless steel (not etched) 
(a) polished; (b) detail of a; (c) b in polarized light 
3.2 COPPER BRAZE 
At the edges of the heat exchanger the stainless steel plates are brazed together and 
there are large gussets of copper braze. Short etching with V2A-pickle only attacks the 
copper phase. The copper in the gusset is very coarse grained and no or only isolated 
grain boundaries are visible (Fig. 5, 6a). In this region copper is also observed in the grain 
boundaries of the stainless steel and a depth of about 50 pm is visible. In comparison with 
the results observed in Fig. 4a it can be assumed that the copper in the grain boundaries 
and not the stainless steel is corroded. Additionally fine grained structures are observed 
within the copper braze and in the copper / stainless steel interface region there is an 
additional phase