Roughness of 1.0721 steel after corrosion tests in 20% NaCl

Abstract Non-alloy quality case-hardening steels are used for low-load components made on automatic machining centers (automatic lines). Because of the widespread use of these steel in open constructions, they are exposed to atmospheric corrosion. The study attempted to analyze the effect of 20% aqueous NaCl solution on the roughness of the steel as a result of corrosion. The steel roughness and corrosion wear were determined according to corrosion time.


Introduction
Non-alloy quality case-hardening steels designed for processing on automatic machining centers have an increased sulfur and phosphorus content. The introduction of these elements affects the properties of steel. During machining, short brittle chips are formed to allow machining of steel on automatic machining centers. Unfortunately, the elevated sulfur and phosphorus content also lowers the mechanical, physical and chemical properties of steel. It does not have high strength or physical properties. Also, it is expected to lower its resistance to aggressive agents, including corrosive agents. Equally important is choosing the right quality material for your product (KOCAŃDA D. ET  Corrosion mechanism of such steel is not as complex as steel with increased corrosion resistance. Its chemical composition and microstructure indicate susceptibility to surface corrosion. Corrosion, though the least dangerous of known corrosion types, causes systematic material destruction by oxidation. Material loses its volume and, therefore, strength and stiffness. Non-alloy quality case-hardening steel, due to low mechanical and physical properties, was not particularly investigated. There are also few publications presenting the results of corrosion resistance tests of this steel. In practice, the problem of changing its surface condition due to corrosion and corrosion resistance is serious, because of its wide application, and seems to be significant. It is also useful to evaluate steel according to the standard for the assessment of corrosion resistant steel (DUDEK A. ET  From the foregoing considerations, it is necessary to carry out corrosion resistance testing of non-alloy quality case-hardening steels in NaCl environment.

Experimental
The experiment was performed on low carbon 1.0721 (10S20) steel designation according to EN 10277-3-2016, with diameter ϕ6.00 mm and 40 mm long.
Before the experiments, samples were successively polished with water paper to R a = 0.32 µm, and next cleaned with water and next with 95% alcohol.
Samples with ferritic and a small perlitic microstructure were tested in accordance with standards dedicated for stainless steel PN EN ISO 3651-1 corrosive media were represented by 20% NaCl.
The corrosion rare of the 1.0721 steel measured in mm/year was calculated with the use of the below formula (1), measured in g/m 2 were calculated with the use the below formula (2): where: t

Results and discussion
The real chemical composition of the tested steel is presented in Table 1.  Arithmetical mean roughness value R a of 1.0721 steel after corrosion tests in 20% NaCl is presented in Figure 1. The regression equations and its determination coefficients r 2 is presented in (3). Mean peak width R q of 1.0721 steel after corrosion tests in 20% NaCl is presented in Figure 2. The regression equations and its correlation coefficients r 2 is presented in (4). Maximum roughness depth R p of 1.0721 steel after corrosion tests in 20% NaCl is presented in Figure 3. The regression equations and its determination coefficients r 2 is presented in (5). Total height of the roughness profile R t of 1.0721 steel after corrosion tests in 20% NaCl is presented in Figure 4. The regression equations and its determination coefficients r 2 is presented in (6). Relative mass loss RML of 1.0721 steel after corrosion tests in 20% NaCl is presented in Figure 5. The regression equations and its determination coefficients r 2 is presented in (7). Corrosion rate measured in mm per year of 1.0721 steel after corrosion tests in 20% NaCl is presented in Figure 6. The regression equations and its determination coefficients r 2 is presented in (8). r corm = 2·10 -6 t -0.001 t + 0.468 and r 2 = 0.9083 (8) Corrosion rate measured in gram per m 2 of 1.0721 steel after corrosion tests in 20% NaCl is presented in Figure 7. The regression equations and its determination coefficients r 2 is presented in (9).

Summary and conclusion
All analyzed surface condition parameters represented by roughness can be represented with a sufficiently accurate firstorder function. This fact confirms that this steel corrodes as a result of uniform corrosion. Evenly increasing roughness parameters show that during the experiment the maximum effect was not reached, after which the corrosion parameters oscillate around a constant size. Corrosion rates measured in mm/year and g/m 2 indicate an increase in the rate of corrosion in the first period. This growth is typical for any material treated with corrosive agents. Taking corrosion time to zero, the corrosion rate tends to infinity, the process starts to reach a constant value over time. In the second period, the process is stabilized and the corrosion rate oscillates around 0.32 mm/year and 0.28 g/m 2 , which also confirms the uniform corrosion course.