Corrosion Assessment of Metals in the Presence of Food Waste Fermentation Solution Through the Experimental Study

This paper describes an experimental study on the corrosion assessment of metals in the food waste fermentation solution. The corrosive degree of metals under different submersion conditions is assessed through the weight loss method. The experimental results indicate that HT200, Q345B, and 45 steel are rapidly corroded (weight loss values over 80, 221 mg/m2 after 192 hours). Although 304 shows a better corrosion-resistant ability, it is slightly corroded (weight loss: 1, 782 mg/m2 after 192 hours) under the interval submersion condition. The food waste treatment facilities, especially pipelines, and bioreactors should avoid iron-based materials.


Introduction
Generally, iron-based materials are widely used for environmental treatment facilities.Some researchers have already found that bacteria and pH drops could cause the metal corrosion problem [1].Normally, the cost of stainless steel is higher than carbon metals.However, considering the mechanical requirements and the manufacturing cost, the materials of food waste treatment facilities should be appropriately selected.However, the corrosive degree of common metals in food waste treatment facilities is not been thoroughly investigated.Therefore, this paper focuses on the corrosion assessment of four common metals (HT200, Q345B, 45 steel, 304) in the presence of food waste fermentation solution.Compared with the experimental results, the corrosive degree of those metals can be determined.

Materials
The food waste fermentation solution is prepared in the following steps (as shown in Figure 1).Firstly, the food waste is collected from the local canteen and it is ground into small particles.The particle size of the mixture is less than 4 mm.Thirdly, the mixture is fed into the fermentation bioreactor and total solid content (TS) is artificially controlled at around 10%.The mass transfer of the biological reaction can be improved by heating and the bioreactor always keeps at a desirable temperature (35 ℃).Sodium hydroxide solution is daily added into the food waste mixture to maintain the pH value between 4 and 6 by sodium hydroxide solution.After 6 days, the food waste fermentation solution is ready for the following experimental study and the physicochemical characteristics as shown in Table 1.Gray iron (HT200), carbon steel (45 steel), high manganese steel (Q345B), and stainless steel (304)) are used and the detailed information is shown in Table 2.All metal blocks are rounded off to minimize the effects caused by the surface roughness.The surface roughness values of all tested metal blocks are measured to be around 0.2 um.

Experimental Setup
Normally, metallic facilities face three operation conditions according to the different exposure levels.
We designed the automatic control device to simulate these different operation conditions.The photo of the automatic control device is presented in Figure 2 and the tested metal blocks are placed on the specified supporting platforms.Here, four blocks of different metals are placed on the No. 1 support platform and fully submerged.Moreover, the No. 2 platform is regulated to the predetermined position, and then half of each metal block is exposed to air.Furthermore, the No. 3 platform is firstly regulated to the lowest position and metal blocks are fully submerged.Then, this platform has risen to the highest position after 30 minutes, and thus metal blocks are totally exposed to air for 30 minutes.After that, the No. 3 platform would be regulated again to the lowest position and repeat the cycles through the PLC controller.

Corrosion Assessment Methods
In this study, the corrosive degree of the tested metal blocks is measured by the weight loss method.Firstly, the tested metal blocks are removed from the supporting platforms after the preset reaction time is reached.Each metal block is cleaned with deionized water and dried for weighting.L ij is defined as the weight loss value for different metals (i= A, B, C, D) under the specific conditions of test (j=1, 2, 3).The calculation is shown below: Weight loss: where M ij is the beginning weight value of the metal i under the j condition; m ij is the weight value of the metal i under the j condition at the preset reaction time; S ij is the surface area of the metal i under the j condition.The higher the weight loss values are, the more serious the corrosion problem would be [3].

Effect of Materials
The weight loss values of different metals under specific conditions are shown in Figure 3.Under the full submersion (FS) condition, the weight loss values of HT200 are highest (14, 050 mg/m 2 after 192 hours).Under the half submersion (HS) condition, the weight loss values of HT200 and 45 steel have nearly the same growth rate before 88 hours, and then the weight loss value of 45 steel (22, 361 mg/m 2 ) is slightly higher than that of HT200 (21, 393 mg/m 2 ) at 192 hours.Moreover, the weight loss values of Q345B are always lower than 45 steel and HT200, but it reaches 19, 720 mg/m 2 after 192 hours.For the interval submersion (IS) condition, HT200, 45 Steel and Q345B show a similar trend, with an increase to around 80, 000 mg/m 2 after 192 hours.Furthermore, the weight loss values of 304 are raised very slightly, no matter which submersion condition it is.Generally, the food waste fermentation solution contains high concentrations of organic acids with chloride [4].HT200, 45 steel, and Q345B contain a high concentration of iron and most iron-based metals can be corroded by organic acids [5].Furthermore, the stainless steel can appear to pitting corrosion in the formic acid and acetic acid solution with chloride.Therefore, in order to avoid the serious corrosion problem, high-level corrosion-resistant materials or polymer composites, such as titanium alloy or glass fiber reinforced composite materials, should be considered as the materials of food waste treatment facilities.

Effect of Submersion Condition
The weight loss values of metal blocks under the different submersion conditions are illustrated in Figure 4.As shown in Figure 4, the rising trend of HT200, 45 steel, and Q345B is very similar.It rises slightly under the FS condition (over 14, 050 mg/m 2 at 192 hours) and increases steadily under the HS condition (over 22, 361 mg/m 2 at 192 hours).Meanwhile, the weight loss values of these metals sharply increase under the IS condition (over 80, 221 mg/m 2 at 192 hours).Although the trend of 304 is different from other materials, the weight loss values of 304 under the IS condition are still the highest (1, 782 mg/m 2 at 192 hours).In theory, for those submersion conditions, the environmental influences mainly come from atmospheric corrosion and microbial corrosion [6].Both are caused by the electrochemical reactions between the metal surface and the conductive medium.The results indicate that the corrosive degree of each metal block is the highest under the IS condition.For HT200, 45 steel, and Q345B, the corrosive degree of the FS is the lowest.Although the corrosive degree of 304 is still the highest under the IS condition, the difference between these submersion conditions is slight.For food waste fermentation solutions, there are diverse microbial communities that directly extract or consume electrons from the metals [7].That is the main reason why the metal blocks quickly corroded in the presence of food waste fermentation solution.Therefore, metallic bioreactors would face more serious corrosion problems if those food waste facilities were exposed to fermentation bacteria and wet air conditioning simultaneously.

Conclusions
This paper carried out the corrosion assessment of metals (HT200, 45steel, Q345B, and 304) in the food waste fermentation solution through experiments.The experimental results show that HT200, 45 steel, and Q345B easily and rapidly corrode because of high iron content and microbial corrosion, especially HT200 and 45 steel.Under the IS condition, the weight loss values of HT200, 45 Steel, and Q345B are over 80, 221 mg/m 2 after 192 hours.Although 304 shows a better corrosion-resistant ability for food waste fermentation solution, it still corrodes in a short time under the IS condition.The weight loss value of 304 is 1, 782 mg/m 2 after 192 hours.Meanwhile, the corrosive degree of all tested metals is the highest under the IS condition.The microbial communities in the food waste fermentation solution can be directly extracted or consumed electrons from the metals, and thus the metal blocks in this study are quickly corroded.According to the experimental results, food waste treatment facilities should avoid iron-based materials.High corrosion-resistant materials, such as glass fiber-reinforced composite materials, should be considered in order to prevent serious corrosion.

Figure 1 .
Figure 1.Flow Chart of Food Waste Fermentation Solution Preparation.

Figure 2 .
Figure 2. Automatic Control Device Used in the Study: 1. Supporting platform, 2. Stepper motor, 3. Encoder, 4. PLC controller, 5. Container of holding food waste fermentation solution.2.3 Analytical MethodsThe samples of food waste fermentation solution are taken from the fermentation bioreactor.The concentrations of total solid (TS), volatile solids (VS), and soluble chemical oxygen demand (SCOD) are measured according to the Standard Methods for the Examination of Water and Wastewater[2].The values of pH and dissolved oxygen (DO) are measured using an analytical mass spectrometer (SL1000, Hach).The weight values of the tested metal blocks are measured by an electronic balance (BSA124S, Sartorius).The surface roughness values are measured by a 3D imaging system (Bruker NPFlex, Bruker), and the surface corrosion images of tested metal blocks are taken by a scanning optical microscope (Axioscope 5, Zeiss).All experiment data are the arithmetic means of triplicate measurements.

Figure 3 .
Figure 3. Weight Loss Values of Different Metal Blocks under the Specific Condition: (a) FS (b) HS (c) IS.

Table 1 .
Physicochemical Characteristics of Food Waste Fermentation Solution.

Table 2 .
Chemical Composition of Tested Metal.