On the corrosion testing of weldments of high alloyed CrNiMo stainless steels and NiCrMo-alloys

Weldments of high-alloyed CrNiMo stainless steels and NiCrMo alloys can be more susceptible to localized corrosion than the solution annealed basic material owing to segregations and precipitations in the heat affected zone, the high temperature zone and/or in the weld. To investigate these differences the FeCl3-test (10% FeCl3 . 6aq), the test ''green death'' (11.5% H2SO4, 1.2% HCl, 1% CuCl2, 1% FeCl3) as well as chronopotentiostatic tests in artificial sea water or in 3% NaCl-solution are used. In particular for testing the highest alloyed materials a CaCl2-test was developed (4.5M CaCl2, chronopotentiostatic test in duration of 8 to 10 hours at + 200 mV (SCE)), which can be carried out to a temperature of 115 degrees C at atmospheric pressure. The aggressivity increases in the range FeCl3-test, ''green death''-test, CaCl2-test. Matching and graduated over-alloyed weldments (TIG, heat input of 7 and 15.5 kJ/cm) of materials 1.4529, 1.4562, 2.4856, 2.4819 (german materials No.) are comparingly examined in various tests, of materials 1.4406, 1.4539, 1.4439 and 1.4563 (german materials No.) only matching weldments in the FeCl3-test. In strongly oxidizing media only a highly over-alloyed performed weldment (filler material 2.4607, german material No.) produces the best corrosion behaviour, measured as the critical temperatures of localized corrosion. Measurements of critical current densities of passivation can be used for investigations of corrosion behaviour of weldments, too. Critical current densities of passivation are showing a tendency to inverse proportion to the critical temperatures of localized corrosion. Suitable electrolytes are among others 0.2M H2SO4+1M NaCl+10-3% KSCN, N-2-bubbled, 25 to 60 degrees C and xM H2SO4 + 4M NaCl + 10(-3)% KSCN (x = 0.05 to 1), 25 degrees C, in con tact with air. An influence of heat input at the welding is indicated in the test of localized corrosion, but it is only small. It is sometimes more clearly shown at measurements of passivation.