ABSTRACT: the traditional free-cutting brass is mostly leaded brass, such as: HPb59-1, HPb63-3 and so on are widely used in machinery manufacturing. Such leaded brass parts will pollute the environment in various forms when they are scrapped. In recent years, with the rising of environmental sound, the development of lead-free free-cutting brass becomes inevitable. However, the corrosion resistance of brass is a very important performance, but also the test can be applied to the actual production of the problem. This paper describes in detail the improvement of dezincification resistance of free-cutting brass and its causes.

Key Words: Free Cutting; Brass; dezincification Chinese picture classification number: TG146.1 + 1; TG291 document identification number: A article number: 1001-4977(2006)05-0516-03 brass corrosion resistance is extremely important service performance. Brass corrodes very little in atmosphere and low speed, dry and pure steam, corrodes not very fast in pure fresh water, corrodes speed slightly to increase in sea water, but in water solution containing o 2, Co 2, H 2s, so 2, Nh 3 and so on gas, the rate of corrosion has increased dramatically. There are two main corrosion forms of brass, namely dezincification corrosion and stress corrosion. And Stress Corrosion can be eliminated by de-stress annealing. The mechanism of dezincification of brass in oxygen-containing neutral salt aqueous solution is generally considered as redeposition mechanism, double-vacancy mechanism, percolation mechanism, and many reports at home and abroad, the dezincification corrosion of brass can be prevented by adding some alloy elements such as arsenic and Boron, and the mechanism of dezincification corrosion of brass has been studied. To that end, we Exploratory research.

1. Test methods and equipment based on lead brass HPb59-1 alloy with Bi instead of PB, and adding a certain amount of rare earth, alloy composition control test. The optimum addition amount of Bi and re was obtained. After melting, casting, testing, hot extruding, drawing and annealing, the lead-free brass bars with good properties were obtained. Specific process flow: batching-melting-analysis-casting-extrusion-testing-stretching-straightening-cutting sample-dezincification test/wear test. Main equipment: 50kg power frequency furnace, 3000k N extrusion machine, 300k n stretching machine, box annealing furnace, straightening machine, constant temperature water bath, line cutting machine, wear-resisting machine, x-ray fluorescence, analyzer, neot-21 metallographic microscope.

2. Test procedure 

2.1 the composition of the free-cutting brass rod obtained in this experiment was analyzed by X-ray fluorescence analyzer as shown in Table 1.

 2.2 dezincification test process and results the sample after extrusion-tension is uniformly processed on the wire cutting machine to form a column sample with diameter 12mm and height (h)10mm. After beating

Grinding and polishing to make the surface smooth and regular. The sample for dezincification corrosion should be set first, then polished and polished. The dezincification corrosion test was carried out in a constant temperature bathtub according to the international standard [8] . The specimen was placed vertically in a beaker containing a standard amount of 1.0% Cu C12 solution (the amount of solution is proportional to the sample area, 240 ml per square centimeter) , with a water bath temperature of 75 °C and a corrosion time of 24 hours. The test equipment is shown in figure 1.

After the corrosion is finished, take out the sample, clean, alcohol clean, blow dry and put it in the dryer. After a number of days, the sample from the center of the axis of symmetry line cut, polished smooth. The thickness of the corrosion layer was measured under a microscope. Some samples of free-cutting brass were compared with HPb59-1 and H62, and the thickness of corrosion layer was measured after magnification of 100 times by metalloscope as shown in figures 2,3,4 and 5.

Table 2 shows the corrosion layer thickness of a typical sample of free-cutting brass in Fig. 2-fig. 5. It can be seen from the table that: (1) The dezincification resistance of HPb59-1 is the worst, but the dezincification resistance of free-cutting brass is generally better; (2) adding

The higher the Zn content in bi and re free-cutting brass, the thicker the dezincification layer.

2.3 results of hardness and Abrasion Resistance Test and preparation of Analytical Specimens: In hardness test, the numbered specimens are processed into cylindrical specimens with diameter 12mm and height (h)10mm. The wear resistant sample is processed into standard sample of 20mm 15mm 12mm. The hardness of the specimen was measured by Brinell hardness Tester (pressure = 250kg, indenter diameter 5mm) , and the wear resistance was measured by wear resistance tester (positive pressure 75kg, rotating speed 400r/min, time 2Min) . The hardness test results of some samples selected for this test are shown in Table 3.

From Table 3, it can be seen that: 

(1) the hardness of the sample is higher than that of HPb59-1, about 121% ~ 201% of lead brass, the wear resistance is much higher than that of HPb59-1, and the wear capacity is about 9% ~ 40% of lead brass; 

(2) the wear resistance of free-cutting Bi brass specimen increases with the increase of copper content. 

3. Mechanism Analysis (1) the alloying elements in brass gradually diffuse to the surface layer with the corrosion, and form a new alloying surface film layer, that is, the alloying elements change the defect structure of cuprous oxide film [10] , thus the copper alloy has a more perfect surface protection film, prevent Zn diffusion and loss. (2) Pickering and Wagner [11] proposed that the anodic dissolution of zinc on the surface first results in a double vacancy, then the double vacancy diffuses into the alloy and the zinc atom diffuses to the surface due to the influence of the concentration gradient, resulting in the preferential dissolution of zinc. The diffusivity of double vacancy in copper at 25 °C is 1.310-l2cm2/S, and that of single vacancy at 25 °C is only 310-l9cm2/S. The diffusion channel of Zn is blocked due to the filling of Bi atoms into the double vacancy. The grain boundary of the brass (Cu-Zn alloy) is strengthened, and the corrosion sensitivity of the grain boundary is greatly reduced, thus the dezincification is restrained and the corrosion resistance is improved. [10] it is proposed that zinc micronutrient inhibition is mainly realized by zinc diffusion inhibition, which is mainly shown in two aspects: one is the formation of a protective film, which prevents the diffusion and loss of zinc; On the other hand, the diffusion Channel 

(3) which is filled into the double vacancy and hinders the diffusion of Zn (3) the addition of re can refine the grain greatly, thus increasing the grain interface and increasing the resistance for the diffusion of Zn. On the other hand, due to the purification and metamorphism of rare earth elements, the impurities, pores and porosity in rare earth brass are reduced, thus the hardness is increased 

(4) in the course of wear, due to the dispersion of Bismuth in the Matrix, the ideal hard point (Bi itself is hard) and the soft matrix structure (+) are formed. During the wear process, when the soft matrix is worn, the hard point forms a bump, which supports the friction surface, thus greatly improving the wear resistance of the material. The morphology of hard particles is round and round, which is beneficial to the good combination of Matrix and prevents the nucleation and propagation of crack. The addition of trace rare earth elements can also refine the grains and make the phase distribution uniform. According to the theory of delamination of wear mechanism [12] , these can release the stress produced by plastic deformation and retard the nucleation and propagation of crack. 

(5) the cutting performance, hardness and wear resistance of the free-cutting brass made in this test are better than those of the traditional lead brass. 

Source: Chinanews.com, by Chen Bingxuan

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