Study On Middle Temperature Phosphating Of Cast Iron

Grey cast iron250and vermicular cast iron340are widely used to produceengines and large castings, which both used to be phosphated to promote anticorrosionproperty. However, owing to much graphite on the surface, they are difficult to bephosphated in traditional ways which seriously affects their application. So it isnecessary to improve craft or invent new technology for phosphating.This paper took grey cast iron250and vermicular cast iron340which were used ina factory as the experimental objects, and made a large amount of experimentalresearches on the phosphate orthogonal test of manganese-znic-calcium sieresphosphating at middle temperature, the relationship between the properties of thephosphating coating and the phosphating conditions. They were observed by SEM andthe phosphating mechanism was theoretically analyzed and discussed, and the optimumphosphating parameter range had been proposed. The technology we obtained has beenapplied for production.The main results are as follows:①For grey cast iron250, by selecting the five factors of total acidity, the ratio oftotal acidity to free acidity, ferrous ion concentration, active time and phosphate time,we choose the orthogonal tables L16(4~5), respectively to investigate the influence of thephosphating parameters upon anticorrosion property. It is found that: the factors whichinfluenced the anticorrosion property were listed by the importance as follows: theconcentration of calcium nitrate, phosphate time, total acidity, the ratio of total acidity tofree acidity and active time. Raise total acidity, enhance the ratio oftotal acidity to freeacidity, reduce the concentration of calcium nitrate, medium active time and appropriatephosphating time can lead to a better anticorrosion property. The parameters which canget the best anticorrosion property are: total acidity50point, the ratio of total acidity tofree acidity18, the concentration of calcium nitrate0.5g/L, active time40s, phosphatingtime10min.②According to the optimum phosphating parameters of the orthogonalexperiments and analysis from experiments, the optimum phosphating parameters ofgrey cast iron250was extended to a certain range, and experiments were carried out toverify the phosphating range, finally the phosphating parameter range were identified:total acidity:40~55point, the ratio of total acidity to free acidity:15~20; theconcentration of calcium nitrate:0.5g/L; acid time:30s; active time:30s; phosphating time:10min. Using these technology, the anticorrosion property of phosphating coatingwhich tested by natural salt spray test could reach more than2h.③For vermicular cast iron340, by selecting the five factors of total acidity, theratio of total acidity to free acidity, ferrous ion concentration, active time and phosphatetime, we choose the orthogonal tables L16(45), respectively to investigate the influenceof the phosphating parameters upon anticorrosion property. It is found that: the factorswhich influenced the anticorrosion property were listed by the importance as follows:the concentration of calcium nitrate, total acidity, phosphate time, the ratio of totalacidity to free acidity and active time. Raise total acidity, enhance the ratio oftotalacidity to free acidity, reduce the concentration of calcium nitrate, appropriate activetime and medium phosphating time can lead to a better anticorrosion property. Theparameters which can get the best anticorrosion property are: total acidity50point, theratio of total acidity to free acidity18, the concentration of calcium nitrate0.5g/L,active time40s, phosphating time10min.④According to the optimum phosphating parameters of the orthogonalexperiments and analysis from experiments, the optimum phosphating parameters ofvermicular cast iron340was extended to a certain range, and experiments were carriedout to verify the phosphating range, finally the phosphating parameter range wereidentified: total acidity:40~60point, the ratio of total acidity to free acidity:15~20; theconcentration of calcium nitrate:0.5g/L; acid time:30s; active time:40s; phosphatingtime:10min. The anticorrosion property of phosphating coating which tested by naturalsalt spray test could reach more than2h.⑤This paper employed SEM to observe the phosphating coating growth process.The coating growth process was divided into three main stages: corrosion of thesubstrate, creation and growth of phosphate crystal nucleus and thickening of phosphatecoating particles. Then the mechanism of phosphatization was discussed.