# Fire Behavior and Adhesion of Magnesium Phosphate Coatings for the Protection of Steel Structures
## 3 Results and Discussion
The temperature of the substrate (steel plate) assessed on the opposite side of the one coated with MPCs vs. the contact time with the flame is presented in Figure 10. As can be seen, for the uncoated steel plate (reference), the temperature of the substrate abruptly increases to 600 \({}^{\circ}\)C in the first 2 min and then increases with a lower rate up to 700 \({}^{\circ}\)C. The substrate temperature of the plates coated with MPCs also presents a sharp increase in the first 2-6 min (see insert in Figure 10), but after that, it stabilizes around 500 \({}^{\circ}\)C. The lowest maximum temperature, around 490 \({}^{\circ}\)C, was assessed for the steel plate coated with MS_Ac_EG. When the fly ash is also present in the MPC formulation (MS_F_Ac), the rate of the temperature increase in the steel substrate is lower as compared to the previously discussed samples, but the maximum temperature achieved is still high (around 500 \({}^{\circ}\)C). The visual aspects and the type of fracture assessed by a pull-off test of the MPC coatings (before and after the direct contact with the flame) are presented in Figure 4:0  
Figure 4:1 Substrate (steel plate) temperature vs. contact time with the flame. Reference—uncoated steel plate. As can be seen, the M cement has a good adhesion to the steel substrate before contact with the flame. According to data from the literature, this good adhesion could be due to a phosphating film which is formed at the steel-MPC interface and can contain FeH(H\({}_{2}\)PO\({}_{4}\))\({}_{2}\) \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{2}{c}{**Before Fire Test**} & \multicolumn{2}{c}{**After Fire Test**} \\  & **Visual Aspect** & **Type of** & **Visual Aspect** & **Type of** \\  & & **Fracture** & **Visual Aspect** & **Fracture** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Visual aspects of the steel plate covered with the studied MTCs, after pull-off test. This test was performed on the MPC coatings before and after the fire test. and FePO\({}_{4}\) as main products [17; 18]. This film, located at the interface between the steel plate and coating layer, also exerts a protective effect on steel corrosion [17]. After contact with the flame, the M coating's failure becomes cohesive, which could be due to an increase in the adhesion strength (Figure 11a). The addition of the polymer suspension (Ac) did not improve the adhesion to the steel substrate; an adhesive failure of the M_Ac coating (at the interface of the coating-steel substrate) occurred before the fire test. After the fire test, the failure becomes partially cohesive, and the adhesion strength increases (see Figure 11a). Several factors could determine this behavior, i.e., the decrease in the mechanical strength of this material (Figure 6) correlated with a lower contraction of this coating compared with the M coating (Figure 11b). The partial replacement of magnesia with waste glass powder (MS_Ac) improves the adhesion strength to the metal substrate before the fire test. The decrease in this material's compressive strength after the thermal treatment (Figure 6) could be an explanation for the cohesive fracture of this coating. For the MPC coating with waste glass and fly ash content (MS_F_Ac), the failure mode is adhesive before the fire test. After the contact with the flame, due to local swelling of the coating in the contact zone with the flame (see also Figure 11b) and probably local delamination, the adhesion strength dramatically decreases (Figure 11a). Figure 11: (a) Adhesion strength of phosphate cement coatings to the steel plate, before and after direct contact with the flame for 1 h, and (b) changes in the thickness of the coatings (in the area which was in direct contact with the flame). Figure 14: Optical microscopy images of coating MS_Ac, before and after contact with the flame (magnification \(\times\)220): (**a,c**) before fire test; (**b,d**) after fire test.