Prakt. Met. Sonderband 38 (2006) 353 
_ooking at Looking at the surface of the heat exchanger plates the grain boundaries of the stainless 
ed. Some steel are visible (Fig. 10a, b). In the LOM a red-brown coverage of the surface is observed 
(Fig. 10a), which can be explained by the formation of red CuO from metallic Cu on the 
steel surface (Fig. 4). Fig. 2 shows that the whole heat exchanger is covered by a thin Cu 
layer. At the water side it becomes red-brown and at the oil side it is still copper coloured. 
For comparison the cross section of a steel sheet is shown, which also indicates that the 
grain boundaries are influenced by increased Cu diffusion (Fig. 10c). 
this case 
ies of the 
je regions 
8.2 wt. %. 
juaternary 
and lattice 
0” cms 
less steel Rss FE 
0]. Fig. 10: Surface and cross section of the steel plates after contact with water medium 
e into the (a) surface in LOM; (b) surface in SEM; (c) cross section 
tes at the 
ade visible 
4. SUMMARY 
A defective plate heat exchanger was investigated. The plates are of stainless steel and 
the braze is copper. The data of the brazing process are not available. A first succession 
was that the water used is too corrosive and attacks Cu. 
After cutting the heat exchanger into pieces no relevant copper and/or steel corrosion is 
visible. 
Metallographic investigations show: 
The large Cu gussets are very coarse grained. Precipitations along the grain 
boundaries in the Cu braze contain mainly Cr and Fe. 
At the interfaces between the large Cu gussets and the stainless steel an additional 
ss steel. phase is observed. EBMA measurements indicate higher amounts of Cr and less Ni 
compared with the original steel composition. An obvious explanation can be the 
formation of a chromium-rich ferrite, which is also confirmed by microhardness 
measurements. 
i case Cu Cu diffuses along the grain boundaries into the stainless steel. In some areas the 
des of the 180 um thick sheets are fully penetrated, because diffusion occurs from both sides 
Iso known of the sheet. 
\n obvious Cu also diffuses into the y-Fe lattice. These regions can be made visible by a 
mperature combined FeCls / V2A-pickle etching. 
In some locations the grain size of the austenitic stainless steel is very large and 
reaches about 180 um. 
a’ 'h -