Multispecies colonisation and surface erosion on A106 GB industry-finished steel used in heat exchangers

Figures & data

Figure 1. Three-dimensional representation of the Atomic Force micrographs of mechanically polished 3 micron (a) and 400 grit (b) finished surfaces.

Figure 2. Representative SEM images of smooth (a) and rough (b) alloy surfaces.

Figure 3. Scanning electron micrographs depicting the smooth surface on day 3 (a), day 6 (c) and day 13 (e) and rough surfaces on day 3 (b), day 6 (d) and day 13 (f).

Figure 4. Scanning electron micrograph of the original etched alloy A (without batch media exposure) revealing the perlite (dark area) and ferrite (light area) phases. The lamellae can be seen in the perlite.

Table 1. Bacterial abundance on the alloy surface determined by rDNA gene sequencing, with top 5 species.

Smooth surface*	Species read count	%^†	Smooth surface*	Species read count	%^†	Smooth surface*	Species read count	%^†
Total early colonisers (day 3)	1425	100	Total middle colonisers (day 6)	9315	100	Total late colonisers (day 13)	9522	100
Clostridium intestinale	804	56.42	Unknown	3979	42.72	DesuIfotomaculum aeronauticum	4548	47.76
Clostridium butyricum	155	10.88	Clostridium butyricum	2427	26.05	Pseudomonas	3293	34.58
Clostridium metallolevans	149	10.46	Clostridium metallolevans	1219	13.09	Unknown	473	4.97
Unknown	123	8.63	Clostridium intestinale	489	5.25	Clostridium butyricum	338	3.55
Streptococcus	50	3.51	Methylobacterium adhaesivum	187	2.01	Sedimentibacter	233	2.45
Other species**	144	10.1	Other species**	1014	10.88	Other species**	637	6.69
Rough surface*	Species read count	%^†	Rough surface*	Species read count	%^†	Rough surface*	Species read count	%^†
Total early colonisers (day 3)	2486	100	Total middle colonisers (day 6)	9002	100	Total late colonisers (day 13)	9767	100
Clostridium metallolevans	986	39.66	Pseudomonas	2816	31.28	Comamonas	8913	91.26
Clostridium intestinale	702	28.24	Unknown	1975	21.94	Desulfotomaculum aeronauticum	161	1.65
Clostridium butyricum	307	12.35	Clostridium butyricum	1830	20.33	Unknown	159	1.63
Unknown	229	9.21	Clostridium metallolevans	854	9.49	Delftia	98	1.00
Pseudomonas	71	2.86	Clostridium intestinale	422	4.69	Psuedomonas	55	0.56
Other species**	191	7.68	Other species**	1105	12.27	Other species**	381	3.90

*The P-value among both groups were determined to be p(perm) >0.05. † Species percentage abundance. **Other individual bacterial species amounting to less than 1 % and attached to the alloy surface.

Figure 6. Scanning electron micrographs after cleaning the corrosion products on the abiotic surfaces, depicting the smooth surface on day 3 (a), day 6 (c) and day 13 (e) and rough surfaces on day 3 (b), day 6 (d) and day 13 (f).

Figure 7. Raman spectra of smooth alloy A after exposure on day 3, 6 and 13, with inserts indicating lepidocrocite (a) and magnetite (b).

Table 2. Mean surface roughness parameters before and after bacterial exposure.

Smooth finish*	After exposure Root mean square Rq (nm)	Rough finish*	After exposure Root mean square Rq (nm)
Day 0	8.68 ± 1.24	Day 0	39.16 ± 15.87
Day 3	114.67 ± 10.69	Day 3	70.70 ± 2.1
Day 6	55.81 ± 11.70	Day 6	260.83 ± 31.35
Day 13	31.25 ± 4.61	Day 13	490.33 ± 121.32

*The p-values among both groups were determined to be <0.05 and Student’s t-test showed significant differences. (N = 3/group). Scan size of 10 µm.

Figure 5. Scanning electron micrographs after cleaning the bacterial biofilm and corrosion products on the surfaces, depicting the smooth surface on day 3 (a), day 6 (c) and day 13 (e) and rough surfaces on day 3 (b), day 6 (d) and day 13 (f). Intergranular corrosion is indicated by the white arrows.

Table 3. Elemental composition of smooth alloy A (without batch media exposure) and after bacterial exposure in the biotic system (with biofilm) on days 3, 6 and 13.

Elements	Alloy A before exposure*	Alloy A after exposure on day 3*(with biofilm)	Alloy A after exposure on day 6*(with biofilm)	Alloy A after exposure*on day 13*(with biofilm)
C	0.19	7.23	12.18	8.52
Fe	98.29	74.60	57.15	58.51
Si	0.26	0.22	0.21	0.17
Mn	0.87	0.00	0.00	0.93
P	0.00	0.33	0.56	0.11
S	0.00	0.24	0.33	1.89
Cr	0.10	0.00	0.00	0.00
Mo	0.10	0.00	0.00	0.00
Ni	0.02	0.24	0.27	0.27
Al	0.03	0.00	0.00	0.00
Cu	0.01	0.00	0.00	0.00
Nb	0.02	0.00	0.00	0.00
Ti	0.01	0.00	0.00	0.00
V	0.03	0.00	0.00	0.00
Cl	0.00	0.02	0.01	0.00
K	0.00	0.03	0.11	0.00
W	0.05	0.00	0.00	0.00
Na	0.02	0.47	0.78	0.00
O	0.00	16.60	28.27	29.60
Ca	0.00	0.03	0.14	0.00
Total	100	100	100	100

*Units indicated in Weight %.

Table 4. Elemental composition of the smooth alloy a of the cleaned abiotic surface (control) and cleaned biotic surface (without biofilm).

Elements	Alloy A abiotic system on day 3* (cleaned)	Alloy A abiotic system on day 6* (cleaned)	Alloy A abiotic system on day 13* (cleaned)	Alloy A after exposure on day 3* (without biofilm)	Alloy A after exposure on day 6* (without biofilm)	Alloy A after exposure on day 13* (without biofilm)
C	7.19	7.10	5.95	8.08	9.94	9.05
Fe	84.66	86.71	90.06	84.65	60.2	46.91
Mn	0.83	0.85	0.97	0.82	0.67	0.61
O	7.32	5.35	3.02	6.45	29.1	43.4
Total	100	100	100	100	100	100

*Units indicated in Weight %.

Table 5. Corrosion rates for alloy A.

Smooth Alloy A	Without bacterial exposure (smooth)*	With bacterial exposure (smooth)*	Without bacterial exposure (rough)*	With bacterial exposure (rough)*
Day 3	0.31 ± 0.04	2.25 ± 0.13	1.02 ± 0.054	1.7 ± 0.02
Day 6	0.32 ± 0.15	−96.5 ± 0.17	0.40 ± 0.02	38.7 ± 0.15
Day 13	0.29 ± 0.03	0.36 ± 0.13	0.24 ± 0.02	1.20 ± 0.05

*Units indicated in mm/y. p > 0.05.

Table 6. Corrosion products formation of alloy a after bacterial exposure.

Corrosion product	Exposure time in days	Wavenumber (cm^−1) observed in this study	Values close to reference below	Notes
Lepidocrocite ($\gamma$- FeOOH)	Day 3 (no Raman peak, deduced from SEM (Figure 5), and day 6, day 13 (most intense bands)	375 cm^−1, 1308 cm^−1	Antunes et al. [31]	Kartsonakis and Charitidis [32], reported values of 393 cm^−1.
Goethite (𝛼 -FeOOH) Mackinawite	Day 13 (Sharp intense peak) and day 6 (not intense).	250 cm^−1	Genchev and Erbe [33] and Leban and Kosec [34]	Kartsonakis and Charitidis [32], reported a value of 273 cm^−1 for geothite. Genchev and Erbe [33], reported 253 cm^−1 for mackinawite.
Magnetite (Fe3O4)	Day 6 and day 13 (very broad)	688 cm^−1, 660 cm^−1	Colomban et al. [35], Antunes et al. [31], Genchev and Erbe [33]	Kartsonakis and Charitidis [32], reported a value of 683 cm^−1.

Figure 8. FTIR spectra of alloy A day 3 showing acetylenic compounds stretching at 2162 cm⁻¹. Day 6 showing the acetylenic compound peak was less intensified (2148 cm⁻¹), and the carbonyl peak was still present at 1948 cm⁻¹. Day 13 showing acetylenic compounds at a stretch of 2162 cm⁻¹, indicating decreased amounts of acetic acid produced by the bacteria.

Table 7. Functional groups.

Origin	Group frequency wavenumber cm^-1	Group frequency wavenumber cm^-1 from literature	Assignment	References
C≡C	2212, 2162, 2183, 2194, 2177, 2148, 2202, 2185, 2162	2260–2100	Acetylenic compounds	Coates [38]
Transition metal carbonyls	2077, 1948, 2085, 1960	2100–1800	Transitional metal carbonyls	Coates [38]

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Data availability statement

Data will be made available upon request through the corresponding author and/or the director of the project, Prof. Evans Chirwa (Email: [email protected]).