The photomicrograph at left is shown at 500X with a Nital etch. Note the black phase (probably FeO) penetrating along the cracks.
Improper feedwater treatment caused the failure. The pearlite has become spheroidized by the boiler's heat.
|
Steel can stress corrosion crack just like copper alloys,
but from different environments. This is a piece of boiler plate
that had an adjacent rivet hole, in which crevice the hydroxyl ion
(OH-) was allowed to build up. The resulting caustic
embrittlement (due to the development of a high pH in the stagnant
water in the crevice) caused extremely localized corrosion and crack
formation in the steel under stress. The photomicrograph at left is shown at 500X with a Nital etch. Note the black phase (probably FeO) penetrating along the cracks. Improper feedwater treatment caused the failure. The pearlite has become spheroidized by the boiler's heat. |
| Now go
on to Specimen 5. |