# Durability Issues and Corrosion of Structural Materials and Systems in Farm Environment
## 3 Corrosion and Degradation of Structural Materials in Farm Environment
### Acid Corrosion Mechanism in Cement Structures  
#### 3.4.1 Inorganic Acids  
The underlying mechanism that dictates the rate of corrosion is dictated by the presence of H\({}^{+}\) and OH\({}^{-}\) ions. Nearly all types of cement are basic (pH \(>\) 12.5) due to the presence of Ca(OH)\({}_{2}\). The high pH of the cement provides a protection against acid attack. The acid lowers the pH and destroys the concrete but the high pH does not exacerbate the attack. Exposure to organic (such as oxalic and acetic acid) or inorganic acids (H\({}_{2}\)SO\({}_{4}\), HCl, H\({}_{3}\)PO\({}_{4}\), and HNO\({}_{3}\)) leads to the lowering of the equilibrium and hydrolytic decomposition as shown in Equation 1 and 2, which impair the properties of the cement structures [18]. In particular, if the pH decreases below 12.5, the Portland cement dissolves [18]. A further decline in the pH leads to the dissolution of cement hydrates (such as calcium alumino-ferrite hydrates and calcium aluminate), which cannot withstand acid/neutral environments. The net effect is the formation of amorphous hydrogels, ferric oxides, Al2O3-aq, and Fe2O3-aq after complete degradation of the original cement microstructure [18]. The final acid attack products illustrate the adverse effects of acid attacks on the durability of concrete structures. The severity of acid corrosion is influenced by time, as illustrated in Figure 4. The samples exposed to acid environments for 3-6 months (AC-3M and AC-6M) were highly corroded compared to those that were exposed for two months or the control (NC) [19]. The phenomenon illustrates there was a positive and definite relationship between the frequency of acid exposure and the durability of concrete structures [19]. Additionally, there were slight differences depending on whether the concrete specimens were exposed to biogenic (bio) or abiotic sulfuric acid; corrosion was most pronounced in the former case. Figure 3: Mineralogical, Chemical and Biological conditions for biotic dominated and abiotic dominated corrosion [17]. “Reproduced with permission from [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON], Water Respublished by Elsevier, 2018.”The corrosion of concrete and steel structures occurs in three primary steps. The first step is characterized by the diffusion of the effluents and other agricultural fluids that are found in farms. The fluids percolate through the pores in the concrete, weakening the structures [18]. The second step is characterized by an advanced chemical reaction between the fluids and the material. The physicochemical properties of the fluids influence the severity of the reaction. For example, acids aggressively attack cement through the mechanisms depicted in Equation 1 and Equation 2, leading to the formation of hydration products and the complete degradation of the concrete structure. The final step is characterized by the formation of soluble and insoluble products following advanced chemical reactions. Additionally, the insoluble products are deposited onto the corroded substances and the diffusion of the corroded items through the soluble layer. \[\text{Ca(OH)}_{2}\text{+}2\text{HA}\rightarrow\text{CaA}_{2}\text{+}2\text{H }_{2}\text{O} \tag{1}\]  
\[\text{xCaO\text{-}ySiO}_{2}\text{-}\text{nH}_{2}\text{O}+2\text{xHA} \rightarrow\text{xCaA}_{2}+\text{ySi(OH)}4+(\text{x}+\text{n}-2\text{y}) \text{H}_{2}\text{O} \tag{2}\]  
Inorganic acids have a similar effect on the structural integrity of acidic structures. The impact of the acid attacks on the surface and cross-sectional morphology of the cement microstructure is visualized in the scanning electron micrographs in Figure 5a,b [15]. The decline in strength was proportional to the duration of exposure to acidic environments, as shown in Table 5. The compressive strength of the control (ordinary pozzolanic cement decreased from 29.4 MPa to 21.23 MPa after 90 days of exposure to nitric acid), as shown in Table 6. The data shows that mass losses increased consistently in acidic environments.