1. Due to insufficient oxidation time, the film layer appears rusty
When the author was assisting an electroplating factory, I heard the customer's reflection on the quality of the oxide film on steel parts, mainly because of poor protection performance and easy rust spots, so I went to the oxidation group to find out what happened. I saw that the workpiece was taken out to be rinsed and dried within less than 10 minutes after entering the oxidation tank during operation . Asked why the oxidation time is so short, the other party said that the workpiece is already very black , and their understanding is that the process should be completed if it is "black". In fact, this understanding is not comprehensive. When iron and steel parts are oxidized , ferric oxide is first formed. The solubility of ferric oxide in lye is very small. When the concentration reaches saturation, it crystallizes, first forms a crystal nucleus, then grows into a crystal, and finally forms a complete film. This process varies with the material of the oxidized parts . For example, high-carbon steel takes about 20-30 minutes, low-carbon steel and alloy steel take about 50-60 minutes. If the oxidation treatment is less than will be taken out. At this time, the actual The upper crystal nuclei are still in the process of growing up and have not yet been connected into pieces, but it is difficult to identify them with the naked eye. At this time, the test with copper sulfate must not pass, so seeing "black" embarrassment cannot meet the quality requirements. .
2. Improper treatment after oxidation will affect the protective performance of the film layer
Due to insufficient understanding of the meaning of post-treatment of oxidized parts, a factory handled the operation in a formal manner. As a result, it not only failed improve the protective effect of the oxide film, but accelerated the corrosion of the oxide film. It is understood that the main reason is that the cleaning is not effective, and the water quality of the prepared soapy water has not been softened, resulting in flocculent floating substances in the solution. Then it accumulates on the surface of the oxide film, and it is the result of wrong practices such as rushing to apply engine oil before it is fully dried. In the end, the oxidized parts have not been sent to the factory and rust has appeared, and they have to be repaired batch after batch. Even more incomprehensible is the failure to recognize the root cause of the rust in this case . After being prompted by the author and making the following requirements, the problem was effectively resolved .
(1) After the oxidation treatment, it should be fully washed with cold and hot water. The oxidation solution is highly alkaline, and it is difficult to effectively clean the lye on the surface of the workpiece by washing with cold water. The blind holes and slits of the workpiece are often still alkaline after washing Wash with hot water to remove alkaline . And because hot water is usually static, the used hot water will show different degrees of alkalinity, so it needs to be rinsed in running cold water at the end.
(2) Pay attention to the three requirements in the saponification treatment process. After the oxide parts are saponified with soapy water, although the protective performance of the oxide film can be effectively improved, it needs to be matched with formal process conditions, the main ones are the following three points .
① The water quality for preparing soapy water must be softened to prevent flocs from appearing in the solution and contaminating the surface of the oxide film.
②The temperature of soapy water should not be lower than 90°C to improve its fluidity.
③The processing time shall not be less than 3 minutes, so as to ensure that the soapy water and the oxide film have sufficient saponification time.
(3) The drying treatment should be effective. Before baking, the residual moisture in the blind holes and slits of the workpiece must be blown out with compressed air , and then baked. The surface of the workpiece and even the holes and slits of the workpiece are not allowed to dry. There is no free water present.
(4) The temperature of hot oil sealing treatment must be guaranteed. The temperature of the hot oil should not be lower than 105°C. Only when the engine oil has good fluidity at a temperature higher than this temperature can it truly fill and seal the pores of the oxide film and achieve the purpose of expulsing the adsorbed water on the surface .
If there is a popping sound during the oil immersion process, it means that there is still water in the holes and slits of the workpiece. At this time, the oil immersion time should be increased until there is no water drop popping sound.
3. Stringing method and five requirements of oxidized parts
Iron wire is the most suitable material for stringing workpieces, because the iron wire has high strength and low price, and will not pollute the oxidizing solution. It is not suitable to use copper wire. The copper wire will replace the copper layer in the stringing treatment during pickling, and the stringing part between the copper wire and the workpiece will stick to the surface of the workpiece due to friction, all of which will affect the normal formation of oxide film there. , and the copper wire will . In the stringing process need to pay attention to the following issues.
(1) Selection of stringing position. When choosing the stringing position, defects such as pockets of water and air pockets should be avoided, and large-area parts should also avoid bonding to each other.
(2) When stringing heavy workpieces, the bearing capacity of the iron wire should be considered. For heavy parts, iron wire with large diameter should be selected. Once it breaks during operation, serious equipment, quality and even personal accidents will occur. For workpieces that still need to be pickled for a long time, more attention should be paid.
(3) Finished parts and workpieces with oxide scales should be strung separately. If the finished parts and the workpieces with excessive scale are mixed together during the stringing process , then the finished parts are soaked in acid at the same time as the workpieces with scales, and the oxide scales on the surface of the workpieces with scales When removing, the original smoothness of the surface of the finished workpiece will inevitably be destroyed, and the accuracy will be affected accordingly.
(4) High-carbon steel, low-carbon steel, and alloy steel should be strung separately. The purpose of stringing different materials separately is because different materials have different requirements for the initial and final boiling point temperature of the solution when oxidized. It is difficult to meet the respective process requirements if they are oxidized at the same time. After being strung separately, the oxidation treatment can be carried out sequentially according to the respective requirements for the temperature of the oxidizing solution.
For example: when the high-carbon steel parts reach the temperature out of the tank, it is exactly the temperature required for the initial oxidation of low-carbon steel or alloy steel. The different requirements of temperature affect the oxidation quality.
(5) The problem of stringing small pieces. Standard parts with large batches and workpieces with small contact areas can , but they must be shaken gently under the liquid (do not shake on the liquid surface to prevent oxidation). To prevent damage), so that the position is changed from time to time, so as not to affect the normal formation of the oxide film on this part due to the mutual bonding.
4. Improper pretreatment before oxidation causes quality failure
The pretreatment requirements of the workpiece are very strict, and its quality requirements are no less than the pretreatment before electroplating. Some people use the " two-in-one " process, that is, to remove the oil on the surface of the workpiece while oxidizing, which is not appropriate. Experience has proved that if the oil on the surface cannot be removed, the quality of oxidation cannot meet the requirements. In addition, the surface of the workpiece has formed a natural oxide film before oxidation , and the black oxide film of ferroferric oxide cannot be formed on the naturally formed oxide film, which eventually leads to serious blooming and even white spots on the surface of the oxidized workpiece. This kind of failure is seen from time to time. A certain electroplating factory ignored this point and caused many batches of oxidized parts to be repaired. Not only did it suffer economic losses, but it also lost its reputation. Two consecutive customers left without saying goodbye.
Therefore, the pre-treatment of the workpiece is very important. It must be fully chemically degreased before entering the pickling process. If necessary must be brushed and electrolytically degreased. A satisfactory oxide film can only be obtained after passing the above-mentioned processes.
5. The method of oxidizing the copper solder joints in the oxidized parts into black
After the copper welding parts are oxidized, the copper solder joints are still yellow, and the workpiece is black and yellow. The contrast is very large, which affects the appearance . At this time, the following methods can be used to make up for it. After the bluish workpiece is fully rinsed, use a brush dipped in brass ammonia liquid oxidant and apply it on the brazing surface, and the place will quickly change from the original yellow to blue-black, reaching a color similar to that of the oxide film. The blue-black copper oxide film formed on the brazing surface will turn red when exposed to high temperature, so the oil temperature should not be too high during oil sealing.
6. Four requirements for maintaining the oxidation solution
The service life of the oxidizing solution depends on the method of use and daily maintenance, among which the following points should be noted.
(1) It should be used intensively. Concentrated use can not only save energy, but also prevent the solution from decomposing .
(2) Prevent the solution from being polluted. Strengthen the pretreatment process of the workpiece to prevent oil and acidic substances from being brought into the tank.
(3) Strictly control the process conditions. When working, prevent the boiling point of the solution from being too high and the solution from boiling endlessly, so as not to cause unnecessary decomposition of the active ingredients in the solution. For this reason, intermittent heating can be adopted in the later stage of oxidation, or the heating power can be reduced to change the solution from violent boiling to gentle boiling, thereby slowing down the decomposition rate of the solution.
(4) Regularly remove impurities in the solution. Before the solution boils, remove the carbonate and its saponification on the liquid surface . After oxidation, remove the iron oxide that sinks to the bottom of the tank before the solution is completely cooled.
7. A simple method for judging the ratio of NaOH to NaNO 2 in the oxidation solution
The concentration of sodium hydroxide ( N a OH ) and sodium nitrite ( NaNO 2 ) in the oxidation solution can be judged by the boiling point of the solution under the normal formula ratio. Concentrations are also higher and vice versa .
However, when the ratio of sodium hydroxide to sodium nitrite is out of balance, it is often difficult to judge, and this imbalance will have a great impact on the quality , but the film layer is loose; when the proportion of sodium nitrite is high, the oxide film formed is thinner but denser. This is a theoretical conclusion. It is difficult to see the difference in thickness and density between the two .
但可通过氧化溶液的颜色大致区别出溶液中氢氧化钠与亚硝酸钠的比例关系，如氢氧化钠比例偏高时，溶液表面呈现青灰色薄膜，当氢氧化钠的比例进一步提高时，青灰色膜层加厚，甚至溶液会出现稀粥状；亚硝酸钠比例偏高时，溶液表面的青灰色逐渐退去，取而代之的是溶液表面出现浅紫红色薄膜，甚至溶液也可能呈浅紫红色。
这一现象可通过烧杯、试管来观察，但对使用过久的氧化溶液就不甚明显了，只能通过相互比较才能识别。
8、 防止氧化膜表面出现红色挂霜
工件经氧化处理后表面有时会附着一层红色沉淀物，这是氧化液使用过久，积有过多的三氧化二铁引起的，这种较少结晶的氧化铁在浓碱液中溶解度很小，因而氧化时会在工件表面沉淀析出，这种现象可采取下列措施予以消除和预防。
（1）严格控制发蓝溶液的工作温度。发蓝溶液所需的工作温度是以钢铁制件的含碳量高低或有无合金成分为According to。含碳量较高的制件，发蓝时溶液的起始温度和终点温度都低一点，而含碳量较低的制件或是合金钢制件则相反。不同种类钢铁制件同时进行发蓝时，时间上要错开，第一槽可先处理含碳量高的制件，待处理到达终点时，溶液的温度也上去了，恰好为含碳量较低的制件起始入槽时需要的温度。这样可避免红色挂霜的出现。
（2）采取二次发蓝工艺。具体做法是：制件先在工作温度较低的溶液配方中进行发蓝处理15min左右，然后将其移至稍高于正常工作温度的溶液中继续发蓝40-60min，第一槽溶液温度控制在130-135℃，第二槽溶液温度控制在140-150℃随钢铁制件含碳量的高低有所区别）。制件在第一槽中形成晶核，然后在第二槽中继续生长，使之连接成片，继而获取致密且较厚具有高耐蚀性的氧化膜，又能大大地减轻红色挂霜的出现。
（3）发蓝过程中增加制件的冲洗步骤。制件在发蓝槽中处理5min之后，从槽中取出来，在自来水龙头下猛冲一下，此时由于制件表面黏附的红色挂霜刚刚形成，较薄，容易冲洗掉，然后再继续发蓝时就较少再会出现红色挂霜。
（4）发蓝溶液中添加亚铁氰化钾。由于亚铁氰化钾能与高价铁离子络合生成普鲁士蓝沉淀，从而降低溶液中铁离子含量，减轻红色沉淀物在制件表面的出现，但此法费用稍高并稍有毒性。
（5）加强发蓝溶液的维护。加强发蓝溶液的维护是防止出现红色挂霜有效的方法之一，是治本的方法，制件表面红色挂霜在一定程度上是随着发蓝溶液中高价铁离子的增加而加重的，为此当发蓝溶液中积聚一定量的高价铁离子之后需要进行清理，具体方法如下：先在发蓝溶液中加入原体积 "1/4’ 的热水，使之稀释，然后加热至沸（此时溶液的沸点温度约120℃(,左右），待静置后吸出槽底红色沉淀物于盆内（待沉淀后上层清液可回发蓝槽），上层清液进行浓缩或补加材料使之合乎工艺配方要求的质量浓度，再进行发蓝其膜层表面将不会出现红色挂霜。此法需经常进行。
(6) Cleared in sodium cyanide solution. The red hanging frost formed on the surface of the oxide film can be removed . After being treated with this solution, the red frost on the surface of the workpiece can be effectively removed. The North and South Chao reminds you to strengthen cleaning .
(7) Manual finger wipe removal. This method is suitable for large pieces. After bluing and soaking in oil, gently wipe the surface of the workpiece with a piece of soft flannelette in an orderly manner, and the red frost will be removed wherever the flannelette goes.
