Corrosion Resistance Investigation Of Materials And/Or Surface Treatments Resistant To Molten Aluminum
In this study, corrosion behavior of 11SMnPb30 + C alloy steel which is not surface treated and boronized at different levels was investigated in liquid aluminum, using liquid ENAC-47000 series alloy.
The use of aluminum alloys has increased in all industries, especially in the automotive industry since they are 66% lighter in terms of density when compared to steel alloys and mainly since the corrosion resistance of alloys that contain magnesium1 is higher.
The use of aluminum increased 9 times in the United States between 1950 and 20092, 40 times in Italy3, whereas it increased by 45-times in China4 only between 1990 and 2009.
This increase in the use of aluminum brought with it production steps such as resting, transporting and molding the liquid aluminum. Liquid aluminum and its alloys are one of the most aggressive liquid metals due to their high chemical activity when compared to almost all metals and metal oxides5. Each year, 2% of steel is corroded by liquid metal and causes billions of dollars of loss8.
The use of liquid aluminum more and more every day in production and the difficulties due to its nature created requirements such as the use materials with high resistance to liquid aluminum, coating and surface treatment. Therefore, in this study, 11SMnPb30 + C alloyed free-cutting steel’s boronizing effect at different levels and the corrosion behavior in liquid aluminum was examined. In addition, the effect of boronizing time to corrosion resistance was investigated.
Research was carried out on nitrocarburizing6 and boronizing7 in order to understand the corrosion mechanism of steel alloys in liquid aluminum and to increase corrosion resistance . In the study carried out by D.C. Lou and et. al in 2005 among these studies, it was concluded that the resistance of steel alloys in liquid aluminum is increased by boronizing.
11SMnPb30 + C alloyed free-cutting steel, which can be found easily and used in pin production, was used to examine the corrosion behavior of boronizing in liquid aluminum. Samples were cut cylindrically 70mm in length and 18mm in diameter. They were cleaned with ethyl alcohol and prepared for boronizing. Ekabor-II (90% SiC, 5% B4C and 5% KBF) powder was used for boronizing. Samples were boronized for 3 and 6 hours at 1000 °C. The resulting samples were cut and examined under an optical microscope. After 3 and 6 hours, the depth of boronizing was determined to be 190 and 250 microns, respectively.
Immersion into Liquid Aluminum
The liquid ENAC-47000 alloy, which is widely used in high-pressure casting due to its high Si content, was preferred for immersion into aluminum. After the samples kept in liquid aluminum for 1 hour are removed, the excess aluminum remaining on their surface was chemically cleaned by standing for 8 hours in 10% NaOH solution.
The cleaned samples were first weighed on precision scales and then were cut by taking circular cross sections, after grinding and polishing, they were etched for 7 seconds in a 2% diluted Nital solution.
The weight difference measurements of the samples on the precision scales were calculated in mm3/cm2 hours and the results were as shown in Chart 1 below. The loss of mass in the non-boronized sample was 0.82 mm3/cm2, while it was respectively measured as 0.27 ve 0.18 mm3/cm2 in samples that were boronized for 3 and 6 hours. Accordingly, as the boronizing time increases, the corrosion resistance against liquid aluminum increases.
These findings support the study of D.C. Lou and et. al in 2005. In this study, 22Cr - 5 Ni Duplex stainless steel and H13 tool steel were boronized for 5 hours at 1025 °C, and cast iron was boronized for 5 hours at 850 °C. Hourly volume losses in liquid aluminum were improved by 20, 3 and 16 times, respectively.
Images obtained from the examinations of the samples with an optical microscope and scanning electron microscope are as shown in Figure 5-7. As the boronizing level increases, a more durable FeB and Fe2B layer forms on the surface and increases the corrosion resistance. In figure 6-a and 6-d boronizing depth, in 6-b and 6-c FeB (black) and Fe2B (grey) can be seen. In figure 7-a and 7-d boronizing depth, in 7-b FeB (black) and Fe2B (grey) and in 7-c surface morphology can be seen.
EDS analysis were conducted to investigate chemical analysis of phases in the boronizing depth and corrosion effect of aluminum. The elements in EDS analysis were limited by Fe, Al and B.
As it can be seen in EDS analysis, Al content is the highest among all other samples and no trace of boron is seen.
Boronizing could not reach in area 4.
Area 1 is rich in Fe2B, area 2 is combination of Fe2B and FeB and Area 3 is rich in FeB. Area 1 is rich in Fe2B, Area 2 contains FeB which is rich in boron and Area 3 is transition area which contains lowest boron.
Hardness tests were carried out with micro vickers (Shimadzu HMV-2 E (334-04109-22)) test at 50-micron intervals from the outside to the inside of the sectioned sample. As expected in samples after boronizing, an increase of hardness up to 6 times was observed. After passing the boronizing depth, hardness was measured as 200 micro vickers similar to the non-processed sample. Measurement results can be viewed in detail in chart no. 2.
As a result of the study carried out, it was observed that the boronizing time had a positive effect on the corrosion resistance of the steel in liquid aluminum. The values found as results of the experiments are compatible with the literature studies. Accordingly, it was confirmed that boronizing is an inexpensive and effective alternative to enhance the corrosion resistance of non-special steels in liquid aluminum.
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