# Comparison of Two Intergranular Corrosion Tests on EN AW-6016 Sheet Material
## 4 Discussion  
The immediate comparison of the micrographs for both applied test methods (ISO 11846:1995-based and PV1113) partially shows a different corrosive attack. ISO 11846:1995-tested samples show primarily an attack along the grain boundaries with some surface grains being etched out. Based on the appearance of the etched-out sections, the complete dissolution of the surrounding grain boundary appears to be likelier than surface corrosion  
Figure 1: Representative micrographs of tested EN AW-6016 material. Three samples each were for the 5 mL/cm\({}^{2}\) (**a**,**c**,**e**) and 6 mL/cm\({}^{2}\) (**b**,**d**,**f**) testing solutions. Two samples were tested with the PV113 (**g**,**h**). as the origin for these solved-out grains. In contrast, PV1113-tested samples show a slightly different corrosive attack. The grain boundaries are attacked as well but the formation of cavity-like features is also observed. These cavity-like features resemble more a pitting corrosion attack. It appears the more potent test solution did cause a more direct corrosive attack on the grain and not just along the grain boundary. This would explain why the IGC depth is smaller compared to ISO 11846:1995 even though the corrosion speed and HCl/NaCl ratio of PV1113 are significantly higher. Comparing the differences in the corrosion speed, a 7-11 times higher speed is seen for PV1113 (Table 4, Figure 2). This difference suggests different potencies of the testing solutions. It is known that the solution amount (Table 4) has an impact on the extent of the corrosive attack [20], but such pronounced differences in the corrosive attack between the two tests must have another origin. A difference in the acid concentration appears to be the main reason for the pronounced differences in the corrosion speeds of both tests. The HCl concentration of PV1113 was determined to be more up to seven times higher than the HCl concentration of the ISO 11846:1995-based tests (Table 4). The NaCl concentration is the same in both tests. It is known that both HCl and NaCl concentration can influence the corrosion speed [3; 21]. The determined HCl/NaCl ratio is up to 11 times higher for the PV1113 test which is almost the same factor observed for corrosion speed (Figure 2). The differences between corrosion speed and HCl/NaCl ratio result most likely from a different type of corrosive attack in PV1113 where not exclusively IGC was observed (Figure 1g,h). PV1113 tests showed the before mentioned cavities which are not recognized by the average IGC depth. It is concluded that by changing the amount of the test solution's acids or electrolyte sources, the reactiveness of an IGC test solution is changed significantly. Taking into consideration the short measurement time of PV1113 with its high temporal tolerance (2 + 0.5 h) compared to ISO 11846:1995 (24 \(\pm\) 0.25 h) and the much higher corrosive speed of PV1113, a higher variance in the test results is usually expected for this test. If enough time is available, a test based on ISO 11846:1995 appears to be more reasonable to produce comparable results. If not enough time is available, PV1113 can be carried out instead, but the testing time of 2 h should be kept with lower tolerance than 0.5 h to minimize systematic test result deviation. Figure 2: Comparison of normalized corrosion speed and normalized HCl/NaCl ratio for all tested samples. In this work, varying test parameters for the ISO 11846:1995-based tests were purposely made. The test solution amount was varied (5 and 6 mL/cm\({}^{2}\) each) and found to have a measurable effect on the test results (Table 5). Up to 25% deviation for the solution amount was determined. This is in good agreement with previous observations where a higher variance of the testing method was investigated (5 and 15 mL/cm\({}^{2}\) each, [20]). The variation of the acid concentration (9, 10, and 11 mL/L) was found to have a measurable effect as well up to 20%.