Knowledge

I. Hazards of Corrosion to Seals

Chemical and electrochemical corrosion pose a serious threat to the service life of mechanical seals. The causes of corrosion are complex and varied. Here, only the most common forms of corrosion in mechanical seals and the most significant influencing factors are analyzed.

 

1.General corrosion and local corrosion

 

Uniform corrosion occurs when the surface of a part in contact with a medium undergoes uniform corrosion. Its characteristic is that the weight of the part decreases, and it may even be completely corroded, losing strength and hardness. For example, when 1Cr18Ni9Ti stainless steel multi-springs are used in dilute sulfuric acid, this situation may occur.

 

Local corrosion can be simply identified by the presence of corrosion spots or holes on the part. Local corrosion causes the surface layer of the part to become soft and porous, prone to flaking and losing wear resistance and strength. Local corrosion is a corrosion form where a certain phase in a multiphase alloy or a certain element in a single-phase solid solution is selectively dissolved by the medium. For instance, when cobalt-based hard alloys are used in high-temperature strong alkali, the binder phase metal cobalt is easily corroded, and the hard phase tungsten carbide skeleton loses strength, resulting in grain flaking under mechanical force. Another example is reaction-sintered silicon carbide, where the surface shows pitting due to the corrosion of free silicon (pH > 10).

 

Corrosion has a significant impact on the performance of seals. Since seals are smaller and more precise than the parts of the main machine, materials that are more corrosion-resistant than those of the main machine are usually selected. For seals that come into direct contact with the medium, although data from relevant corrosion manuals can be referred to for selecting appropriate materials, these data may not be consistent with the usage conditions in mechanical seal systems, as they are mostly corrosion data for a single medium under static conditions, while the medium in the process flow is a mixture of multiple media. Experience shows that pressure, temperature, and sliding speed can all accelerate corrosion. The corrosion rate of seals increases exponentially with temperature.

 

When dealing with highly corrosive fluids, external or double-end face seals should be used to minimize the impact of corrosion on the seals, as they have the fewest parts in contact with the process fluid. This is one of the most important principles for selecting seal structures under strong corrosive conditions.

 

2. Stress corrosion

Stress corrosion is a corrosion phenomenon that occurs in metallic materials under stress in a corrosive environment. Whether it is external load or residual stress, corrosion will be intensified. Materials prone to stress corrosion include austenitic stainless steel, copper alloys, etc. The process of stress corrosion generally begins with the formation of selective corrosion grooves on the metal surface, followed by continuous local corrosion, and finally, cracks develop from the bottom of the grooves under the action of stress. A typical example is the drive sleeve of the 104 mechanical seal, which is made of 1Cr18Ni9Ti. When used in an ammonia pump, the drive ears of the sleeve are most likely to develop stress corrosion cracks, causing damage to the ears. To prevent this stress corrosion, the concave ears were changed to solid convex ears.

 

3. Erosion

The high-speed movement between the seal and the fluid causes microscopic unevenness on the contact surface. When the fluid is a corrosive medium, it will accelerate the chemical reaction on the sealing contact surface. This reaction can sometimes be beneficial and sometimes harmful. If the formed oxide layer is damaged, corrosion will occur. The material damage caused by the alternating action of wear and erosion is called erosion. Generally, erosion does not quickly reflect the change in sealing performance on non-primary components of mechanical seals such as spring seats, thrust rings, and ring seats, but it is one of the main forms of failure of the friction pair. Therefore, in strong corrosive media, the friction pair should use materials with good corrosion resistance, such as 99.5% high-purity alumina ceramics or hot-pressed sintered silicon carbide without free silicon.

 

4. Crevice Corrosion

When there is a very small gap between a metal and a metal or a non-metal component and the medium is in a stagnant state, it will accelerate the corrosion of the metal in the gap. This type of corrosion is called crevice corrosion. For example, the grooves or corrosion spots that appear between the mechanical seal spring seat and the shaft, and between the compensation ring auxiliary sealing ring and the shaft (of course, fretting wear also exists here) are typical examples. The reason for this is that the medium in the gap is in a stagnant state, making it difficult for the substances involved in the corrosion reaction to be replenished into the gap, and the corrosion products in the gap are also difficult to diffuse out. As a result, the medium in the gap becomes increasingly different from the overall medium in terms of composition concentration and pH value as the corrosion progresses, leading to accelerated corrosion of the metal surface in the gap. Crevice corrosion poses a significant threat to the sealing performance. Grooves formed at the sealing ring and the corner shaft will prevent the compensation ring from moving axially, losing its tracking ability, causing the end face to separate and leak. For crevice corrosion, it can usually be mitigated by proper material selection and reasonable structural design. For example, materials with good resistance to crevice corrosion should be selected, and the formation of gaps and stagnant liquid areas should be avoided as much as possible in the structural design; a self-flushing method can be adopted for circulation to keep the medium in the sealing cavity constantly renewed and flowing, preventing changes in the composition concentration of the medium; for long-term idle pumps and machines, the accumulated liquid should be drained in time, etc. It is impossible to completely eliminate gaps in the structure, so protective sleeves are generally used, and corrosion-resistant materials can be sprayed on the installation part of the sealing ring to prevent it.

 

5. Electrochemical Corrosion

 

In fact, all the various corrosion forms of mechanical seals are more or less related to electrochemical corrosion. For the mechanical seal friction pair, it is often subject to the hazard of electrochemical corrosion because the friction pair is usually made of different materials. When they are in an electrolyte solution, due to the inherent different potentials of the materials, an electrochemical couple effect occurs when they come into contact, that is, the corrosion of one material is promoted while the corrosion of the other is inhibited. For example, when copper and nickel-chromium steel are paired and used in an oxidizing medium, nickel-chromium steel undergoes ionization decomposition. Salt water, seawater, dilute hydrochloric acid, dilute sulfuric acid, etc., are typical electrolyte solutions. Sealing components are prone to electrochemical corrosion, so it is best to choose materials with similar potentials or pair ceramics with filled glass fiber polytetrafluoroethylene.

 

II. Failure of Rubber Sealing Rings

For mechanical seals, synthetic rubber O-rings are commonly used as auxiliary sealing rings. Approximately 30% of mechanical seal failures are caused by the failure of O-rings. The failure forms are as follows.

1.A ging

High temperatures and chemical corrosion are usually the main causes of rubber products hardening and cracking. Rubber aging is manifested as rubber becoming hard, with reduced strength and elasticity. In severe cases, cracking may occur, resulting in the loss of sealing performance. During storage and handling, rubber may age if it is exposed to sunlight for a long time, comes into contact with ozone, or is stored for too long. Overheating can cause the decomposition of rubber components, and even carbonization. In high-temperature fluids, rubber has the risk of further vulcanization, eventually losing its elasticity and causing leakage. Therefore, it is necessary to understand the safe operating temperature of each type of synthetic rubber.

 

2. Permanent deformation

Permanent deformation of rubber sealing parts is usually more severe than that of other materials. For instance, rubber O-rings may become square-shaped during use. When sealing rings are exposed to high temperatures for a long time, they will take on the same cross-sectional shape as the groove. If the temperature remains constant, they can still perform the sealing function; however, when the temperature drops, the sealing rings will quickly contract, creating a leakage path and causing leakage. Therefore, it is necessary to pay attention to the temperature limits for various rubber types and avoid using them at the limit temperatures for extended periods. If the sealing operation conditions cannot be changed, structural improvements should be made to reduce the adverse effects of temperature on rubber materials. For example, larger cross-sectional rubber O-rings should be selected as much as possible, O-rings should be kept away from the friction pair end face, the hardness of O-rings should be appropriately increased, and a groove-type assembly structure should be adopted (instead of a push ring compression type structure, and the spring force should not act on the O-ring).

 

3. Swelling deformation

Synthetic rubber may expand, become sticky or dissolve in certain media. Therefore, the appropriate material should be selected based on the nature of the working medium by referring to the relevant data and charts. If the composition of the working medium being conveyed is not very clear, an immersion test should be conducted to guide the rational selection of materials. Some mixed solutions may corrode various synthetic rubbers, in which case polytetrafluoroethylene should be used for the sealing ring.

 

4. Distortion and extrusion damage

Rubber O-rings in rectangular grooves of compensating rings may twist and distort during assembly or use. The reasons for this include: low hardness and small cross-sectional diameter of the O-ring, uneven circular cross-sectional diameter, fluctuating working pressure, shock and vibration, as well as low internal pressure and poor lubrication, all of which can cause the O-ring to twist. The twisted part is mostly in the middle of the O-ring. When the twist is severe, the cross-sectional area at that point will become thinner, and at the same time, the leakage and friction force will increase. The methods to prevent the O-ring from twisting are as follows:

① Before installation, apply lubricating grease in the groove and ensure the shaft is smooth to allow the O-ring to roll freely.

② The compression amount should be as appropriate as possible, and the width of the groove should be appropriately increased to allow the O-ring to roll in the groove.

③ When several cross-sectional sizes are available, larger cross-sectional O-rings should be preferred.

④ Use other sealing rings that do not twist, such as X-shaped cross-sectional sealing rings.

 

Rubber O-rings are always in a compressed state under static and displacement motion conditions, so there is a tendency to extrude into the gap under high-pressure conditions. O-ring extrusion means that the O-ring under high pressure will cause stress concentration at the gap, and when the stress reaches a certain level, the O-ring will form a burr that embeds into the gap, resulting in wear or damage to the O-ring, premature failure of the seal, and leakage of the medium from the sealing ring. Clearly, the main causes of extrusion are related to pressure and the gap at the sealing location, as well as the hardness of the O-ring material. Reducing the gap can prevent extrusion, but it will reduce the floating and following characteristics of the sealing ring. Therefore, in high-pressure conditions, the measure to prevent the extrusion of rubber O-rings is to install a retaining ring in the O-ring groove. Especially for small cross-sectional O-rings, a retaining ring made of polytetrafluoroethylene or polyimide should be added.​​​​​​​

 

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