Multi-component boron coatings on low carbon steel AISI 1018

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• Electroless liquid <strong>boron</strong>izing. The common salt bath is 30-70 % Borax-B 4C
and the reaction is enhanced by replacing 20 % B 4C with ferroaluminium, which is an
effective reductance. For the nickel alloy, the bath is composed of 74 wt. % KBF4 -25
wt. % KF, operating at temperatures below 670 °C.
• Electrolytic liquid <strong>boron</strong>izing. The specimen material is attached to a cathode,
while the graphite is acted as an anode. Both of them are immersed in the molten borax
at 940 °C and the current (0.15A/cm) is passed to them. At the liquid state, Borax is
decomposed to sodium ions (Na+) and tetraborate (B40 72). The reaction is as followed:
At anode; B4072- —> B407 + 2e - —> 2B203 + 0
At Cathode; 6Na+ + B 203 —> 3Na2O + 2B.
2.3.4 Gas Boronizing
The gas <strong>boron</strong>izing process is a diffusion process of some gas media, such as diborane
(B2H6) or <strong>boron</strong> chloride (BCI 3). This process is unpopular for the industry because these
gas media are toxic and explosive. The BCI3-H2 gas mixture has previously been
attempted to <strong>boron</strong>ize steels, but a high concentration of BCI3 caused the corrosion on the
tested substrate and resulted the poor-adherence layers. To improve the process
practicality, the diluted (1:15) BCI3-H2 gas mixture is commonly used at 700- 900 °C and
under the pressure of about 67 kPa [9]. Moreover, the usage of a carrier-reductance gas
of 75%N2 and 25%H 2, rather than H2, can reduce the BCI 3 concentration. As a result, it
avoids FeCl3 (corrosion) and diminishes the FeB formation. In addition, this process can
be used with titanium and its alloys.