*2.2. Gravimetric Measurements*

The gravimetric experiments were carried out in a 1.2 L high-pressure autoclave (PARR instrument) at different CO2 partial pressures (1, 20, and 40 bar). The coupons were suspended in a 1.0 L solution

in the presence and absence of different concentrations of the inhibitor (i.e., from 0.6 up to 2.0 g L<sup>−</sup>1) at 25 and 60 ◦C. Before each experiment, the tested solution was deaerated with CO2 for 2 h under atmospheric pressure and then CO2 was purged for another 2 h at the tested pressure after the introduction of the samples. After saturation, the pH and conductivity of the tested solution were 4.5 and 60.30 mS cm−<sup>1</sup> at 1 bar and 25 ◦C, respectively. To ensure homogeneous mixing, a Teflon-coated blade agitator was used (e.g., 200 rpm). The weight loss was determined by retrieving the coupons after 24 h of immersion by means of an analytical balance with an accuracy of ±0.1 mg. To assess the effect of time, the samples were immersed for 168 h in the presence and absence of 1.0 g L−<sup>1</sup> of GA at different CO2 partial pressures (1, 20, and 40 bar). The corrosion products were removed according to the ASTM G1-90 [30], then the specimens were ultrasonically washed with distilled water, dried with absolute alcohol, and reweighed. In each case, the experiment was conducted thrice and the corrosion rate (*CR*) in mm y−<sup>1</sup> was obtained from the following equation:

$$CR\left(\text{mm y}^{-1}\right) = \frac{87.6\text{ }\Delta m}{dAt} \tag{1}$$

where, Δ*m* is the weight loss calculated form the difference between the initial (*W*i) and the final (*W*f) weight (mg). d is the density (7.87 g cm<sup>−</sup>3), *A* is the surface of the sample (cm<sup>−</sup>2) and *t* is the immersion time (h). The inhibition efficiency (*IE*%) was determined using the following equation [20,22,26]:

$$
\Delta I \text{\%} = \frac{\text{CR} - \text{CR}^{\text{inh}}}{\text{CR}} \times 100 \tag{2}
$$

where *CR*inh and *CR* are the corrosion rates of the steel with and without the inhibitor, respectively.
