3.1. Potentiodynamic Polarization Test of Statically Corroded Uncoated and Coated SS AISI 316L
The Tafel plots and calculated potentiodynamic polarization results of the SS substrate and the coated specimens are presented in Figure 3
and Table 3
, respectively. The Ecorr calc
of the bare AISI 316L after 10 min of immersion were found to be −0.423 V vs. SCE and 1.9 µA∙cm−2
, respectively. Ecorr calc
shifts toward more positive value (to about −0.340 V vs. SCE) after applying the AlCrN PVD coating on the SS substrate. About 1.2 times lower icorr
was measured as compared to the AISI 316L, reaching improvement in the protective efficiency (Pi
) by 15.9%.
A significant passivation of the AlCrN PVD coated sample was found after immersion for 24 h. Ecorr calc significantly shifts toward a positive value (−0.153 V vs. SCE) and icorr was found to be ≈30 times lower (0.05 µA∙cm−2) as compared to the coating after 10 min immersion. The icorr was also found being ≈40 and ≈450 times lower as compared with the tested substrate after 10 min and 24 h immersion, respectively.
Decrease in CR
down to 4.9 × 10−4
down to 0.002% for the AlCrN PVD coated sample was detected after 24 h immersion. It confirms a hydrolyzation reaction with the AlN on the surface and inside the pores. One of possible reaction is known as the Bowen’s model [23
] reporting degradation of the AlN in an aqueous medium. In the present work, the reaction passivates as the AlN is embedded into a monolithic dense coating and the reaction occurs only on the surface. Corrosion products such as an amorphous aluminium monohydroxide (AlOOH(amorph)
) and a crystalline aluminium hydroxide (Al(OH)3(crystal)
) form an additional passivation layer. Increased Pi
up to 99.8% confirms the inertness of transition metal (e.g., Cr) nitrides [24
3.2. Tribo-Corrosive Wear Test of Uncoated and Coated SS AISI 316L
Evolution in OCP before, during and after the short (2 h) wear tests of the bare and coated SS are presented in Figure 4
a. Loss of protective oxide film developed on the surface of the bare SS was continuously observed during the whole stabilization period (1000 s) before starting the wear test. OCP shifts towards negative values from about −0.400 down to about −0.680 V vs. Ag/AgCl during stabilization.
Sharp shifting towards more negative values of OCP was measured at the beginning of wear test. Stabilized OCP value of about −0.780 V vs. Ag/AgCl remains practically unchanged until the end of the test. A sharp increase in coefficient of friction (CoF) from about 0.10 up to about 0.74 in the first 1200 s of the wear test (running-in period) indicates a rapid formation of a wear track representing a removed protective oxide layer and an enlarging tribocontact area. Instability in CoF was observed as a fluctuation from 0.70 to 0.74, exhibiting arrangement of the tribosystem (second running-in period). A slightly higher, but stable CoF of about 0.79 was measured after 2600 s of the test running, which indicates reaching a steady-state regime.
Some fluctuations in OCP from −0.230 to −0.160 V vs. Ag/AgCl were observed with the AlCrN PVD coated steel before wear test. These fluctuations are related to the solution penetration into the pores, other defects presented on the surface of the coatings, and reaction with the available AlN, causing a formation of the passive layer that consists of Al-based reaction products. Simultaneous evolution during the first 1600 s and stabilization in OCP and CoF values of about −0.210 V vs. Ag/AgCl and 0.58 were observed corresponding to the running-in and the steady-state regime, respectively. At these particular conditions, neither failure nor observable degradation of the coating was detected.
The test duration was increased up to 12 h with the application of 1 or 3 kg load as shown in Figure 4
b. The fluctuations in OCP from −0.320 up to −0.250 V vs. Ag/AgCl were generally observed with the AlCrN PVD coated steel before wear test initiation, Figure 4
b. Evolution of CoF during a running-in period (about 4000 s from the beginning of wear test) is similar to the evolution of CoF at 1 and 3 kg loads. An increase in CoF was measured from 0.42 up to the steady-state value of 0.69. A second CoF stabilization period was observed after about 21,000 s of test running, changing CoF to the final steady-state value of 0.65 (1 kg) and 0.62 (3 kg).
A passivation effect of the coated specimen was detected during the test under 1 kg load. The OCP slightly changed from −0.240 up to a more positive value of −0.150 V vs. Ag/AgCl. A rapid change to more negative OCP value of −0.300 V vs. Ag/AgCl was detected after 9000 s of sliding. It can be explained by an increased rate of a continuous mechanical destruction of the passivating layer. A slight passivation effect occurs after about 25,000 s as OCP was measured to be changing from −0.300 up to the final steady-state value of −0.280 V vs. Ag/AgCl.
Sharp shifting in OCP toward more negative values from −0.240 down to −0.440 V vs. Ag/AgCl was observed after the first 3300 s of the test under 3 kg load as shown in Figure 4
b. This value remained stable for about 18,000 s of sliding indicating an inability of corrosion products to create a stable passivation layer under this load. However, the critical failure of the coating was recognized after about 18,000 s of the test run. The OCP was measured to be almost continuously shifting towards negative values from −0.440 down to about −0.700 V vs. Ag/AgCl at the end of the test.
It was also found that passivation effect of the immersed AlCrN coating occurs after about 45,400 s of static oxidation as demonstrated in Figure 5
. The stabilized OCP is more positive than during tribo-corrosion tests (Figure 4
No typical iron based oxides on the pristine surface of the SS AISI 316L were found in the Raman spectra, Figure 6
a. However, the Raman peak of Cr2
with a low intensity at 310 cm−1
indicates the development of the oxide layer at the ambient conditions. An increase in intensities of peaks collected from the area of a wear scar of the pristine SS points out to the formation of a thick layer of iron-based oxides and hydroxides [7
]. A broad peak was observed in a range between 680 and 700 cm−1
indicates the presence of the corrosion product Fe3
, Figure 6
a. It should be noted that almost identical spectrum was obtained even after 24 h of static sample expose into the NaCl solution. Well-pronounced broad peaks appear in the spectra of the AlCrN PVD coated SS at 300, 690–706, 1000 and 1331–1388 cm−1
, Figure 6
b. These peaks belong to the vibration of Cr and N ions and their intensities decrease after immersion in the 3.5 wt % NaCl solution for 12 h. The peak at 1000 cm−1
disappears, but a broad peak at 1331–1388 cm−1
turns into new peaks of low intensities as demonstrated in Figure 6
b. It could be explained by the low and medium intensity combination of Raman active modes and overtones of α-Cr2
on the surface of coated material [25
The typical grooves in the wear scar of the AISI 316L SS after the short tribo-corrosion test are demonstrated in Figure 7
a. Several areas of the CrN rich interlayers and micro-droplet inclusions were exposed during an extended (12 h) tribo-corrosion test under 1 kg load, Figure 7
b. Several areas of the uncovered CrN interlayer and extensive cracking in the middle region were found after the extended wear test under 3 kg load, Figure 7
c. EDS analysis indicated a critical decrease in an atomic content of Al and appearance of elements typical for AISI 316L in the delaminated areas. The simplified schematic illustration of the static and dynamic corrosion processes is made based on the results obtained, Figure 8
. The overview assignment of the Raman peaks of the uncoated and AlCrN coated SS AISI 316L before and after the corrosion and tribo-corrosion tests are presented in Table 4