First, make it clear: the "corrosion" of non-metallic expansion joints is not the same as metal corrosion
When many people hear "corrosion", what comes to mind is rust, electrochemistry and pitting grooves. But corrosion of non-metallic expansion joints is another matter entirely. Non-metallic expansion joint, which is often called fabric fiber expansion joint or non-metallic compensator, is mainly made of polymer materials such as fiber fabric, rubber and polytetrafluoroethylene (PTFE). They have no free electrons, so there's no talk of electrochemical corrosion. The so-called "corrosion" is actually the erosion, swelling and dissolution of chemical media, or thermal oxygen aging and hydrolysis at high temperature.
Metal is like an iron plate, and corrosion is nibbled into it bit by bit from the surface; The non-metal is more like a sponge, and the acid and lye seep in, causing the fiber to decay in strength, delaminate bulging, and even pulverize overall. The failure mode is different, and the solution is naturally different. If you use the set against metal corrosion-thickened walls and top coating-against non-metallic expansion joints, you will be off the beaten track.
2. Acid, alkali, salt or high-temperature flue gas? The corrosion mechanism of non-metallic compensator is different in different media
The diluted sulfuric acid condensate in the desulfurization flue and the alkali steam at the tail of the cement kiln have completely different damage paths to the non-metallic compensator. Strong acids such as sulfuric acid and hydrochloric acid mainly attack the chemical bonds of fiber fabrics, especially the silicon-oxygen bonds in glass fibers, which will gradually hydrolyze and break under acidic environment. Alkali has little effect on PTFE, but it will seriously corrode glass fiber and silica gel coating, making it brittle and cracked.
Let's talk about salt solution. Sulfite and sulfate, which are common in desulfurization system, will crystallize and expand in the alternating dry and wet areas, physically bursting the coating. There is also high temperature flue gas, above 260°C, the PTFE liner begins to decompose to release hydrogen fluoride, which in turn exacerbates acidic corrosion. So "non-metallic expansion joint corrosion" is never a single cause, media, temperature, phase (gas/liquid/solid) must be looked at together.
Third, from fabric fiber to PTFE lining: if the material is selected wrong, no matter how thick the coating is, it will be useless
The corrosion resistance of non-metallic expansion joints depends on material selection for 90%, and the remaining 10% depends on structural design. The common belt materials on the market are nothing more than the following: PTFE (polytetrafluoroethylene), which is resistant to strong acids and alkali, and has a temperature resistance of more than 200℃, but the price is high and the interlayer peel strength is low; Silicone coated glass fiber cloth, high temperature resistance, good elasticity, but not strong acid resistance; Fluororubber, acid resistance is better than silica gel, but it is afraid of high-temperature water vapor; Polypropylene, polyethylene, etc., are cheap but have a temperature resistance of only 100℃.
Take the desulfurization flue as an example. The flue gas temperature is between 80-150℃, and it contains a large amount of condensed sulfuric acid solution. At this time, the inner layer must be protected with PTFE or PFA (perfluoroalkoxy resin), with glass fiber reinforced layer in the middle, and the outer layer must be protected with fluororubber or silicone. If you choose an ordinary rubber compensator to save trouble, it will pierce in three months. On the other hand, in the working condition above 300℃ at the end of the cement kiln, PTFE can't bear it, so it has to be replaced with high silica fiber or ceramic fiber composite structure.
There is no "panacea" for material selection, only "suitability or not". Therefore, experienced buyers will clearly write the media composition, concentration, pH value, dew point temperature and instantaneous overtemperature value when proposing the technical agreement. If it is not written clearly, the manufacturer can only report according to the conventional configuration, and the result is "buy cheap and use expensive".
Fourth, corrosion is often not isolated: stress, temperature and flow rate are all not helping
In addition to chemical attack, non-metallic expansion joint failure is often superimposed with mechanical factors. Under pressure and displacement, the fiber itself has stress. Molecular chains in areas where stress is concentrated are more likely to be broken by the medium, which is called "stress corrosion cracking"-although the term derives from metals, similar phenomena exist in non-metals.
The chemical reaction rate approximately doubled for every 10 °C rise. The same medium may last five years at 80℃, but not one year at 120℃. The flow rate is also a big variable. When the flue gas carries dust particles, it will cause erosion and wear on non-metallic surfaces. After the coating is worn off, chemical corrosion follows, which is the typical "grind first and rot later". Therefore, when professional manufacturers select non-metallic expansion joints, they will consider predicting displacement, fluid velocity, particle concentration and temperature gradient at the same time. If one is missing, problems may occur later.
V. On-site cases: where are desulfurization flue, cement kiln tail and power station air duct planted respectively
Let's start with the desulfurization flue. In a thermal power plant project, the non-metallic expansion joint was used for less than a year, and the PTFE in the inner layer of the skin bulged and cracked in a large area. After removing it, the problem lies in the fact that the flue gas baffle door is not closed tightly. When the furnace is shut down for maintenance, the residual acid accumulates at the bottom of the expansion joint and is soaked for two whole weeks. This "immersion condition" is not usually written at the time of design, but it just happens in reality. The solution is to require manufacturers to add drainage holes at the bottom of the expansion joint, or adopt a double-layer structure, so that even if the inner layer is damaged, it will not directly leak to the outer layer.
Look at the cement kiln tail again. The problem here is the double attack of alkali vapor and dust. The expansion joint of the kiln tail smoke chamber of a cement plant was originally made of glass fiber cloth + silica gel coating structure. As a result, after half a year's operation, the surface coating was corroded by alkali solution, and the glass fiber was quickly powdered after being exposed. After replacing it with ceramic fiber + fluororubber composite ring belt, the problem is solved. The difference is not in the thickness, but in the chemical resistance of the material.
The air duct of the power station is more concealed. The flue gas temperature of the expansion joint at the outlet of the induced draft fan is about 120℃, and the sulfur content is not high, but a large number of pinholes appear on the surface after one year of operation. The test found that the alkaline substances in fly ash formed a micro-battery effect in a humid environment-note that this is not electrochemical corrosion, but the alkaline particles formed a high-concentration lye micro-region after absorbing moisture on the fiber surface, and the local corrosion rate was extremely fast.
6. Anti-corrosion is not an aftermath remedy. These accounts should be calculated clearly in the design stage
Most non-metallic expansion joint corrosion accidents, to put it bluntly, are not settled in the selection stage. Which accounts?
- Media Full Ingredient Billing: It is not enough to write the word "smoke", but SO₂, SO₃, NO, HCl, dust concentration, moisture content and dew point temperature.
- Temperature dynamic account: How many degrees are normal working conditions? How many degrees when driving to stop? Is there an instantaneous overtemperature? How many minutes can overheat last?
- Spatial structure account: Is there any dead-spot fluid? Will there be condensation backwards? Is the expansion joint installed in the horizontal or vertical section? Different locations have completely different structural designs.
- Lifetime economic account: Which is cost-effective: cheap materials are changed in two years, or expensive materials are changed in eight years? Counting downtime losses, the answer is often clear.
In addition, the application standard of non-metallic expansion joint can refer to JB/T 12235-2015, which has clear test methods and technical requirements. When selecting, pay attention to whether the manufacturer designs and tests according to this standard.
In the case of anti-corrosion, whether the materials can bear it or not, and whether the working conditions are selected correctly or not, it is never a choice between two. Find out the working conditions before talking about the selection of materials, and the order cannot be reversed. Basically, each of the above cases exists at the same time as "the working conditions are not thoroughly understood" and "the materials are not selected correctly". The answer to non-metallic expansion joint corrosion lies between the two, depending on which link you want to spend your energy on.