CONCEPT
•L1. Inner Corrosion-Biogenic
        H2SO4
•L2. CaSO4-Microorganisms
        (SOM and heterotrophs)
•L3. Air/Corrosion interface
Sewer head space
bulk H2S conc.
corrosion products
and microbial
population
 H2SO4 conc. profile
H2S conc. profile
1
2
3
Concrete
substratum
HETEROTROPHS
SOM
Ref: Mehta, 1983
MICROSCOPIC VIEW OF INNER
SURFACE OF CONCRETE
THIS SLIDE SHOWS WHAT IS HAPPENING AT THE MICROSCOPIC LEVEL:

•IN LAYER 3, H2S IS PRODUCED BY SULFUR REDUCING BACTERIA.
   -H2S IS ABSORBED INTO THE CONCRETE SURFACE.
•IN LAYER 2, SULFUR-OXIDIZING BACTERIA TRANSFORM THE H2S INTO H2SO4, WHICH ENTERS LAYER 1 AND CAUSES CORROSION.
•BY-PRODUCTS (ETTRINGITE AND GYPSUM) OF CONCRETE CORROSION ACCUMULATE IN LAYER 2.
   -IN LIVE SAMPLES, LESS CALCIUM IONS ARE RELEASED TO LIQUID MEDIA.  THE CALCIUM AND SULFATE IONS JOIN TO FORM GYPSUM ON THE CONCRETE SURFACE.     THIS CAUSES THE CONCRETE MASS TO REMAIN CONSTANT, WHICH IS SEEN IN MY DATA OF % MASS LOSS.
   -IN CONTROL SAMPLES, CALCIUM IONS ARE RELEASED MORE FREELY TO THE LIQUID MEDIA BECAUSE THERE IS LESS SULFATE PRODUCED TO FORM GYPSUM    WITH   CALCIUM.  THIS CAUSES THE CONCRETE MASS TO DECREASE.