Study On Low Alloy Steel Designed For Grounding Grids And Its Corrosion Behavior In Acidic Soil

A functional relationship between corrosion current density of metal and the physicochemical properties of soil such as porosity, moisture content, salt content and pH value was modeled on the basis of simulated soil. The model analysis showed that the corrosion current density descended with increasing porosity; while increasing moisture content, the corrosion current density firstly ascended then descended; with the salt content ascended, the corrosion current density firstly ascended and then stabilized in a relatively high value; the decreasing pH value ascended the corrosion current density. To give a faster assessment of corrosion resistant ability of steel in acid soil, a simulated corrosion evaluation method based on diatomite soil was established, and the results of single factor simulated soil corrosion test agreed well with the predicted trends of simulated soil corrosion model. Based on the previous research, simulated soil A with4:6of the ratio of coarse-grained diatomite particle to fine particle,35%of moisture content,4.5of pH value, and5times of actual soil salt content was established. A comparative study of corrosion behaviors of Q235steel in actual soil and simulated soil indicated that corrosion mechanism in the two soils were similar, and simulated soil A possessed a higher speed-up ratio, of which was about5times as that in actual soil.In order to resolve the corrosion issue of grounding grids steel, a design solution of acid soil corrosion resistant low alloy steel for grounding grids was given based on integrated computing of multiple software tools. Using HSC Chemistry6, JmatPro and Thermo-Calc software, the patterns of carbon, chromium, silicon and manganese contents affecting corrosion-resistance, electrical conductivity and heat fusion joint of steel were analyzed:a moderate content of chromium could effectively inhibit the dissolution of iron anode and expand the passivation area of iron; with the increase of carbon, chromium or silicon contents, the electrical resistivity ascended and the melting point descended. While the electrical resistivity and melting point was less affected by increasing manganese content. Based on integrated computing results of material combination properties, the alloy design of acid soil corrosion resistant steel was less than0.1%of carbon content, around1.0-2.0%of chromium content, less than0.1%silicon content and other elements contents in a moderate value. Low carbon Cr-alloyed Al steel and ultra-low carbon Cr-alloyed A2steel were refined using high-purity process metallurgy. The testing results showed that compared with Q235steel, the Al steel had a0.07μΩ·m higher electrical resistivity and a13℃lower heat fusion joint, of which the electrical conductivity and heat stability were insignificantly deteriorated; while A2steel had a0.014μΩ·m lower electrical resistivity and15℃higher heat fusion joint, and its electrical conductivity and heat stability were better.The corrosion behavior of experimental steels in simulated acidic soil were investigated using weight loss, scanning electron microscopy, energy spectrum scanning, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical techniques. The result indicated high-purity process metallurgy reduced the level of inclusions in steel, minimized the amount of corrosion pitting source; low-carbon design eliminated the corrosion microcells composed of ferrite and pearlite in the typical steel; the addition of appropriate amount of chromium increased corrosion potential of steel, lowered the sensitivity of pitting corrosion. With the development of corrosion, the added chromium enriched in inner rust layer and generated Fe2CrO4, which facilitated the formation of Cr-goethite in steel. Due to the presence of dense aggregation of ultrafine crystals with cation selectivity, the Cr-goethite in rust layer provided the Cr-containing steels an excellent corrosion resistant performance. At the end period of corrosion, the corrosion rate of A1and A2steel were0.58mm/a and0.38mm/a separately, the corrosion resistance of which were30%and50%times higher than that of Q235steel.