Why do metal expansion joints need to be pickled inside? — Talk about those "invisible hidden dangers"
Many people think that the metal expansion joint is a section of corrugated pipe, which can be used by welding the flange. Where is there so much attention? If you really think so, you will suffer a big loss sooner or later. I met a customer two days ago, for a power stationCorrugated expansion joint for power station industryIt cracked and leaked after less than half a year of operation. After removing it, the inside of the bellows is full of rust pits and welding slag, and no internal treatment has been done at all. Tsk, this isn't saving costs, it's laying mines.
During the manufacturing process of metal expansion joint, welding, machining and heat treatment will leave oxide scale, welding slag and oil stain. If these things are left in the inner walls, especially the corners of the bellows valleys, and steam, smoke or corrosive media are passed behind them, they become a hotbed of stress corrosion. If you think about it, bellows walls are inherently thin, and once pitting penetrates, the whole expansion joint is wasted. SoInternal pickling of metal expansion jointIt is not an option, but a life-saving item-only by removing all the invisible rust, oxide scale and oil film and giving the base metal a clean surface can we talk about subsequent passivation protection.
Disassembly of pickling process: from degreasing to passivation, no step can be saved
Degreasing, pickling, passivation. Degreasing is the first step. The grease and cutting fluid on the bellows and guide cylinder wall must be cleaned first, otherwise the acid liquid will not contact the metal surface and dry for nothing. The degreaser generally uses alkaline solution or special cleaning agent, the temperature is controlled at about 60℃, and the circulation rinses for 10-15 minutes.
Pickling immediately after degreasing. Pickling liquid is commonly used as nitric acid-hydrofluoric acid mixture. The concentration, temperature and time should be adjusted according to the grade of stainless steel. For example, againstHigh temperature axial expansion jointCommonly used 304 or 316L, nitric acid concentration 10%-15%, hydrofluoric acid 1%-3%, temperature 40-50℃, soak for 15-25 minutes. The oxide scale will not be removed if the time is short, and it will be over-corroded if the time is long. There is a pit here: in order to speed up, many people increase the temperature or concentration of acid solution, and as a result, the surface of bellows tends to have intergranular corrosion, which is not worth the loss.
Water wash to neutral immediately after acid washing, followed by passivation. Soak nitric acid solution (20%-30%) at room temperature for 20-30 minutes to form a dense chromium oxide film on the surface. This step determines the upper limit of subsequent corrosion resistance. After passivation, rinse with deionized water and dry. Note: The residual liquid must be thoroughly emptied, especially the trough and the gap between the guide tube and the bellows. The residual acid will continue to corrode and lead to later leakage.
Will pickling hurt the bellows? — — Deflectors and the protection strategies of veterans
"Will pickling make the bellows thin?" This is a question asked by every purchaser. The answer is: Yes, if you mess around. Regular pickling only removes oxide scale and slight matrix, and the thickness loss is controlled within 0.01-0.02mm, which has little effect on the fatigue life of bellows. But if you use the wrong acid (such as washing stainless steel with hydrochloric acid), or it takes too long, then the bellows will really become "thin skin and big filling".
The protection measures are divided into two pieces. First,guide tubeIs the key. Many expansion joints are equipped with guide tubes, which function to guide the flow direction of the medium and protect the bellows from being washed. During pickling, the guide tube separates the acid liquid from the inner wall of the bellows to avoid the acid liquid directly soaking the trough. However, the guide tube itself should also be pickled, and the welding place should also be treated. Secondly, for those without a guide tubeUniversal corrugated expansion jointWhen pickling, the liquid level and flow rate should be controlled, and circulating spraying instead of soaking should be adopted to reduce the contact time between the bellows and the acid liquid. In addition, immediately after pickling, it is blown dry with compressed air to prevent the residual liquid from forming "acid pits" in the trough.
How to check and accept after pickling? Don't miss the weld, color, and residual liquid
- Weld area: The oxide scale of the weld and heat affected zone must be completely removed to reveal a uniform metallic luster. If there is local blackening or residual weld slag color, it means that the pickling is not in place.
- Overall Color: The surface of stainless steel after pickling and passivation should be uniform silver white or light gray, without flower spots, blue spots or red rust. In order to save trouble, some manufacturers only pickling without passivation, and the surface begins to turn yellow after a few days, which is a failure.
- Residual liquid detection: Use pH test paper or chloride detection tool to test the drainage and internal dead corners. The pH value should be between 6-8, and the chloride ion content should not exceed 25ppm, otherwise the residual liquid will induce pitting.
- Roughness comparison: Compare with standard photo samples. After pickling, the surface roughness should be uniform, and there should be no "orange peel"-like lines formed by large-area over-corrosion.
By the way,Internal pickling of metal expansion jointThe acceptance is best completed in the factory, and don't wait until the site installation to find it. At that time, the disassembly and assembly cost is enough for you to buy several new expansion joints.
Find a manufacturer to do it or do it yourself? — Work out the economic accounts
Some units feel that they have the qualification of pickling, so they buy barrels of acid and soak them themselves to save money. But have you calculated the combined cost?
Hidden costs of doing it yourself:Acid procurement, waste liquid treatment (environmental protection departments are staring at it), personnel training, safety protection (hydrofluoric acid is highly toxic, and there are many cases of amputation splashed on the skin), and rework caused by uncontrollable quality. Especially forLarge diameter thick wall expansion jointOrStraight pipe pressure balanced expansion jointThis kind of large parts, the internal structure is complicated, and it is difficult to ensure that the dead corners are cleaned by pickling themselves.
Advantages of finding professional manufacturers:They have constant temperature pickling tank, circulating filtration system, waste liquid recovery unit, and process parameters are verified. More importantly, professional factories can provideInternal pickling of metal expansion jointIssue the test report and certificate of passivation film. This money cannot be saved.
A chemical plant bought 8 setsPTFE-lined hoseAnd 4 setsNon-metallic expansion jointAfter pickling and installation, three units leaked three months later. After disassembly and inspection, it was found that the trough residual liquid was corroded. The cost of rework plus the loss of shutdown is enough to buy two new equipment. If I had spent more money to let the manufacturer do it all at once, there would be no such shit at all.
Write at the end: Pickle is not thorough, the expansion joint will show you color sooner or later
Pickling seems simple, but it is actually an integration of craft and experience. There are no standards and specifications, no process control, and no acceptance means, that is to gamble on luck. And in the pipeline system, everyMetal expansion jointAre safety weaknesses — it's thin, it withstands displacement, it touches the medium. The internal pickling gave it its first line of defense. If this line of defense was broken, the rest of its life cycle would be full of hidden dangers.