Chapter 3: The Effect of Nitrite and Nitrate Supplementation on the Corrosion of Mild Steel
3.3 Results
3.3.4 Scanning Electron Microscopy and Electron Dispersive X-ray analysis of Corroded mild
3.3.4.1 Scanning Electron Microscopy
Observations of the mild steel coupons immersed in the loam soil system over a 24 week period were conducted using SEM. Figure 3.2 shows the SEM image of mild steel coupons incubated in the autoclaved and non-autoclaved systems after 4, 8 and 20 weeks of incubation. The autoclaved control displayed no microbial growth over the course of the 24 week incubation period, however, corrosion product formation was observed to a small degree. The formation of nodules was observed at week 4, indicated by the arrow (Figure 3.2 A1). At week 8, a heterogeneous layer of corrosion products was observed to cover the surface of the coupon. Towards the end of the incubation period, the formation of deeps pits and cracks on the surface is seen to occur, surrounded by corrosion products, as shown in Figure 3.2 A3. Microbial growth was not detected in the non-autoclaved system as well, however, the formation of various tubercles, indicative of microbial activity, was observed over the course of the incubation period (Figure 3.2 NA1, NA2, NA3).
Figure 3.3 shows the surface of the autoclaved and non-autoclaved systems treated with nitrite.
Neither microbial activity nor corrosion product formation is observed in the autoclaved and non- autoclaved systems treated with nitrite. Significant corrosion products are not observed in either the autoclaved or non-autoclaved systems throughout the incubation period, in contrast to the non-supplemented systems, however, light corrosion product formation was observed as well as the early phases of pitting corrosion.
Figure 3.4 shows the surface of the coupons incubated in autoclaved and non-autoclaved loam soil supplemented with nitrate. Corrosion was found to be much more severe and occurred at a much higher rate when compared to the non-supplemented and nitrite treated systems, with abundant biofilm growth also observed. At week 4, severe corrosion in the form of deep cracks, as well as sponge-like corrosion product formation, was already seen in the autoclaved system, although microbial growth is not displayed. This corrosion was found to cover the entire surface of the coupon by week 8, in which a thick, heterogeneous layer of corrosion products was observed. Week 20 displayed abundant biofilm growth in the form of cocci shaped bacteria.
Corrosion products were found to be interspersed within the biofilm matrix. The non-autoclaved system supplemented with nitrate displayed very similar characteristics when compared to the
69 autoclaved nitrate treated system. At week 4, spindle-like corrosion product formation as well as deep cracks in the surface are observed although no microbial growth was seen. However, at week 8, abundant non-uniform biofilm formation is demonstrated by the presence of numerous rod shaped bacteria. The bacterial cells were found to be surrounded by corrosion products. As with the autoclaved system, by week 20, severe biofilm and corrosion product formation was found to cover the entire surface of the coupon.
Figure 3.2: Some typical examples of SEM images of the surface of mild steel coupons in a stimulated loam soil system after various weeks of incubation. (A1, A2, A3) – Autoclaved system after 4, 8 and 20 weeks, respectively; (NA1, NA2, NA3) – Non-autoclaved system after 4, 8 and 20 weeks, respectively.
A1 A2 A3
NA1 NA2 NA3
70 Figure 3.3: Some typical examples of SEM images of the surface of mild steel coupons in a stimulated loam soil system after various weeks of incubation. (ASN1, ASN2, ASN3) – Autoclaved system supplemented with 20 mM nitrite after 4, 8 and 20 weeks, respectively;
(NSN1, NSN2, NSN3) – Non-autoclaved system supplemented with 20 mM nitrite after 4, 8 and 20 weeks, respectively
Figure 3.4: Some typical examples of SEM images of the surface of mild steel coupons in a stimulated loam soil system after various weeks of incubation. (AN1, AN2, AN3) – Autoclaved system supplemented with 20 mM nitrate after 4, 8 and 20 weeks, respectively; (NN1, NN2, NN3) – Non-autoclaved system supplemented with 20 mM nitrate after 4, 8 and 20 weeks, respectively
ASN2 ASN3 ASN1
NSN3 NSN2
AN3 AN2 1
1 AN1
1
NN31 NN2
1 NN11
NSN1
71 3.3.4.2 Energy Dispersive X-ray analysis of mild steel coupons
The different corrosion products formed on the surface of both the untreated and treated mild steel coupons were examined using EDX analysis for their elemental composition as described in Section 2.3. The various different types of corrosion products analysed amongst all systems are shown in Figure 3.5, and the elemental composition of these corrosion products is shown in Table 3.3.
The untreated mild steel coupon was mainly composed of iron (88.22 %), carbon (4.87 %), and oxygen (3.53 %), with trace amounts of manganese, fluorine, aluminium, and silicon (0.2%, 1.78%, 0.4% and 1.01% respectively) also present. All corrosion products were composed of varying percentages of iron and oxygen, indicating the occurrence of iron oxidation, as well as carbon, with varying degrees of trace minerals also present.
Corrosion product 1 (IC1) was a biofilm detected in the autoclaved nitrate treated system, with corrosion products interspersed within the biofilm. IC1 mainly consisted of 61.84 % iron and 30.84% oxygen, with increased amounts of carbon (5.22%) compared to the untreated coupon.
Trace amounts of Mn, K, Ca, Ti, Cr, Co, Cu and Ta were also detected but were absent on the untreated coupon surface.
Corrosion product 2 (IC2) was a thin, tightly adhered corrosion product detected in the non- autoclaved control. IC2 was observed to be composed of 61.53 % iron and 32.43 % oxygen, similarly to IC1, however, lower amounts of carbon were present (3.27 %) when compared to IC1 and the untreated coupon surface. Mn was not detected in IC2, however, trace amounts of Si, Ca, Co, Ni, Br and Tb were detected.
Corrosion product 3 (IC3) was a sponge-like, porous, thick, voluminous corrosion product detected on the surface of the non-autoclaved mild steel coupon surface. It was found to contain a similar elemental composition to IC2, with iron (59.49%), oxygen (35.56%), carbon (3.66%) and Si (0.63%) detected in similar amounts. IC3, however, contained Mn (0.07%), as well as higher amounts of Co and Ni and trace amounts of Ca.
Corrosion product 4 (IC4) was detected in the coupons incubated in the autoclaved loam soil. It consisted of spindle shaped corrosion products arranged in a manner resembling a cotton-ball
72 structure. IC4 contained the highest oxygen content (41.13%) and lowest iron content (50.56%) when compared to the other corrosion products examined. However, the carbon content (4.69%) was similar to the untreated mild steel surface. An increase in manganese content (2.9%) was observed in IC4, possibly indicating the activities of metal reducing bacteria. Phosphorous was detected in trace amounts (0.09%), which could indicate the presence of microbial cells. Trace amounts of Ca and Si were also observed in IC4.
Figure 3.5: Corrosion products investigated using EDX analysis. IC1- Corrosion product 1 (Autoclaved 20 mM nitrate supplemented system- week 12), IC2- corrosion product 2 (Non- autoclaved system- week 8), IC3- corrosion product 3 (Non-autoclaved system- week 8), IC4- corrosion product 4 (Autoclaved-system- week 16).
IC1 IC2
IC3 IC4
73 Table 3.4: Elemental composition of different corrosion products formed on the surface of the treated and untreated mild steel coupons analysed by EDX analysis
Sample
Untreated Coupon Corrosion product 1 Corrosion product 2 Corrosion product 3 Corrosion product 4
Element Wt%
Fe 88.22 61.84 61.53 59.49 50.56
O 3.53 30.84 32.43 35.56 41.13
C 4.87 5.22 3.27 3.66 4.69
Mn 0.2 0.15 - 0.07 2.9
F 1.78 - - - -
Al 0.4 0.06 - - -
Si 1.01 0.17 0.63 0.63 0.22
K - 0.13 - - -
Ca - 0.13 0.49 0.2 0.41
Ti - 0.07 - - -
Cr - 0.16 - - -
Co - 0.16 0.18 0.32 -
Cu - 0.16 - - -
Ni - - 0.06 0.08 -
Ta - 0.7 - - -
Br - - 0.31 - -
Tb - - 0.1 - -
P - - - - 0.09
Samples: Corrosion product 1 (Autoclaved 20 mM nitrate supplemented system- week 12), Corrosion product 2 (Non-autoclaved system- week 8), Corrosion product 3 (Non- autoclaved system- week 8), Ccorrosion product 4 (Autoclaved-system- week 16)
74 3.3.5 DGGE analysis of biofilm communities on corroded mild steel coupons and