The Corrosion of Polymeric Materials
Polymeric materials are not corrosion-proof. Preventing failure involves grasping the complexities of corrosion in these materials.
Polymeric materials have wide applications; therefore, there are many factors that can lead to corrosion in these materials. The lifetime of a polymeric material cannot be accurately foreseen in a specific corrosive atmosphere, and so it is necessary to clearly understand the compositions and reaction mechanisms of polymeric materials.
Because polymeric materials don't experience specific corrosion rates, they're typically either fully corrosion-proof against a selected corrodent (within specific temperature ranges), or they deteriorate quickly. They're attacked either by chemical processes or by solvation. Solvation causes the swelling, softening and supreme failure of polymeric materials. Therefore, it is vital to grasp the complexities of corrosion in polymeric materials.
What Are Polymeric Materials?
First, let's take a look at what polymeric materials are and how they behave. Polymeric materials are materials that are made of polymers. They are materials composed of huge molecules, usually based on carbon, that are formed from the chemical bonding of smaller units (monomers).
Polymeric materials are often plastics, but also include elastomers. They are used to make perishable foam, renewable plastics, and even films and coatings. Several polymeric materials were developed to be used in the automobile industry, as well as for clothing and medical applications. Polymeric materials show up in everyday items like milk jugs, tires, medical instruments and edible coatings. Based on their cross-linking network, polymeric materials are classified as:
- Thermoplastics, which don't contain cross-links. These soften upon heating and are repeatedly reshaped.
- Thermosets, which are densely cross-linked. These soften gently and ultimately degrade upon heating.
- Elastomers (also known as rubbers), which are gently cross-linked networks that are deformed by modest force. (For some example applications, read Case Studies of a Novel Elastomer for Cavitation Resistance.)
Classification of the Corrosion of Polymeric Materials
According to their attack mechanism, the corrosion of polymeric materials is often classified in the following ways:
- Disintegration or degradation of a physical nature owing to absorption, permeation, solvent action or various factors
- Oxidation, wherever chemical bonds are attacked
- Hydrolysis, wherever ester linkages are attacked
- Thermal degradation involving depolymerization and probably repolymerization
- Dehydration (rather uncommon)
- Any combination of the above
The result of such attacks can include softening, charring, crazing, delamination, embrittlement, discoloration, dissolving or swelling.
The corrosion of polymer matrix composites is additionally plagued by two different factors: the nature of the laminate and, within the case of thermosetting resins, the cure. Improper or poor cure can adversely impact corrosion resistance, whereas correct cure time and procedures can usually improve corrosion resistance.
Polymeric materials in outside applications are exposed to weather extremes that may be extraordinarily injurious to the material. The foremost harmful weather element, exposure to ultraviolet (UV) radiation, will cause embrittlement, fading, surface cracking and chalking. When polymeric material is exposed to direct daylight for a period of years, it usually exhibits poor impact resistance, reduced overall mechanical performance, and changes in appearance.
Because ultraviolet radiation is definitely filtered by air lots, inclement weather, pollution and other factors, the quantity and spectrum of natural ultraviolet radiation exposure is extraordinarily variable. Since the sun is lower within the sky throughout the winter months, it's filtered through a larger atmosphere. This creates two vital variations between summer and winter sunlight. During winter months, a lot of the damaging short wavelength light is filtered out. As a result, materials sensitive to ultraviolet radiation below 320 nm would degrade only slightly, if at all, throughout the winter months.
Photochemical degradation is caused by photons or light-breaking chemical bonds. For every sort of attractive force, there's an important threshold wavelength of sunshine with enough energy to cause a reaction. Light of any wavelength shorter than the threshold will break a bond, while longer wavelengths cannot fracture it. Consequently, the short wavelength cut-off of light supply is of crucial importance. If a specific polymer is only sensitive to ultraviolet radiation light below 290 nm (the solar cut-off point), it'll never experience chemical deterioration outdoors.
The ability to resist weathering varies with the type of polymer and among grades of a specific resin. Several resin grades are obtainable with UV-absorbing additives to boost weatherability. However, the higher grades of a resin usually exhibit higher weatherability than the lower molecular weight grades with comparable additives. Additionally, some colors tend to weather better than others.
All materials are somewhat porous to chemical molecules, but plastic materials tend to be an order of magnitude larger in their porosity than metals. Gases, liquids or vapors can permeate polymers. Permeation could be a molecular migration through microvoids, either within the polymer (if the polymer is more or less porous) or between polymer molecules. In neither case is there any attack on the polymer. This action is strictly a natural phenomenon. However, permeation is harmful once a polymer is applied to line piping or instrumentation. In lined equipment, permeation may end in:
- Failure of the substrate as a result of corrosion.
- Bond failure and blistering ensuing from the buildup of fluids at the bond once the substrate is less porous than the liner, or from a corrosion/reaction product if the substrate is attacked by the permeative.
- Loss of contents through the substrate and liner as a result of the ultimate failure of the substrate. (In unbounded linings, it's vital that the house between the liner as well as the support member be vented to the atmosphere to permit minute quantities of permeating vapors to flee and to stop the growth of entrapped air from collapsing the liner.)
Different polymers experience different rates of permeation; some polymers aren't plagued by permeation at all. The fluoropolymers are significantly affected. There's no relationship between permeation and the passage of materials through cracks and voids, although in each case, migrating chemicals travel through the polymer from one aspect to the opposite.
The thickness of a lining affects the permeation rate. For general corrosion resistance, thicknesses of 0.010 to 0.020 inches are typically satisfactory, counting on the mix of the liner material and also the specific corrodent. Once mechanical factors like dilution to cold run, mechanical exploitation, and infiltration rates are a consideration, thicker linings may also be needed.
The rate of permeation is affected by:
- Temperature and pressure
- The concentration of permeative
- The thickness of the polymer
The density of the polymer can have an impact on the permeation rate. The thickness can usually decrease permeation by the square of the thickness. The more dense the sheet, the lower the permeation rate. The rate of permeation is also affected by the temperature and gradient within the lining. Lowering these can cut back the speed of permeation. Lined vessels like storage tanks, which are used in closed conditions, give the most effective service.
Polymers have the potential to soak up varied amounts of corrodents, especially organic liquids. This will end in swelling, cracking and penetration of the substrate of a lined part. Swelling will cause softening of the polymer, introduce high stresses and cause failure of the bond on lined elements. If the polymer has a high absorption rate, permeation is likely to happen.
Several steps can be taken to cut back absorption. Thermal insulation of the substrate can cut back the gradient across the vessel, thereby preventing condensation and the absorption of fluids. This reduces the speed and magnitude of temperature changes, keeping blisters to a minimum. The use of operative procedures or devices that limit the ratio of process pressure reductions or temperature can provide additional protection.
Corrosion of Various Polymeric Materials
There are three general categories of polymers:
1. Thermoplasts (Thermoplastic Polymers)
Thermoplasts are long-chain linear molecules that are shaped by heat and pressures at temperatures higher than a vital temperature referred to as the glass transition temperature. Many of the properties and chemical resistance variations of polymers stem directly from the kind and arrangement of atoms within the polymer chains. Of explicit importance and interest among thermoplasts is the class called halogens, which are found within the non-metal class. Of all the halogens, fluorines are the most negative, allowing them to powerfully bond with carbon and hydrogen atoms, but not very well with themselves. The fluorine acts as a protective shield for alternative bonds of lesser strength in a polymer chain. Besides these, the arrangement of components within the molecule, the symmetry of the structure, and therefore the polymer chains’ degree of branching are vital because of the specific components contained within the molecule.
Polymeric materials containing carbon-hydrogen bonds like polypropene and polythene, and therefore the carbon-chlorine bonds like PVC and gas chlorotrifluoroethylene, are totally different from a completely fluorinated compound like Teflon. The latter encompasses a much wider range of corrosion resistance.
One of the applications for plastic materials is to resist atmospheric corrosion. By having the ability to resist attack by specific corrodents in plant operations, they're also able to resist corrosive fumes in the atmosphere. Thermoplastic materials are joined by either solvent cementing, thermal fusion or by means of adhesives. The most important disadvantages of solvent cementing are the likelihood of stress cracking in the parts, and therefore the potential hazards of using low-vapor point solvents. It is tough to pick out an adhesive that may not degrade in two widely differing chemical environments. In general, the adhesives that are most immune to high temperatures typically exhibit the simplest resistance to chemicals and solvents.
2. Thermoset Polymers
Thermoset polymers assume a permanent form or set once heated. As a result, they can't be reformed or recycled. Thermosets are amorphous polymers. These resins are liquid at room temperature. Then, by adding a catalyst or accelerator, they become a rigid product that sets or cures into its final form. The thermosetting resins are high-molecular-weight polymers that are strengthened with glass or different suitable materials to produce mechanical strength. The most commonly used resins are the vinyl esters, epoxies, polyesters and furans. For reinforcing these polymers, fibrous glass of F and C grades is most ordinarily used.
Unreinforced, unfilled thermosetting polymers will corrode by many mechanisms. This sort of corrosion may be divided into two main categories: physical and chemical. Physical corrosion is the interaction of a thermosetting compound with its setting so that its properties are altered without a chemical reaction. The diffusion of a liquid into the compound is an example. In several cases, physical corrosion is reversible; once the liquid is removed, the initial properties are restored. Once a compound absorbs a liquid or a gas leading to plasticization or swelling of the thermosetting network, physical corrosion has taken place.
Chemical corrosion takes place once the bonds within the thermosetting are broken by means of a chemical process with the polymer’s surroundings. It is sometimes irreversible. As a result of chemical corrosion, the compound itself is also affected. For instance, the compound is also embrittled, softened, charred, crazed, delaminated, discolored, dissolved, blistered or swollen.
All thermosets are attacked in a similar manner. However, certain chemically resistant varieties suffer negligible attack or exhibit considerably lower rates of attack underneath a good range of severely corrosive conditions. Curing the resin plays a very important role in the chemical resistance of the thermosetting. Improper activity can lead to a loss of corrosion-resistant properties.
Some environments might weaken primary and/or secondary compound linkages with ensuing depolymerization. Alternative environments might cause swelling or microcracking, whereas still others might change ester groupings or linkages. In some environments, repolymerization will occur, causing a modification to the structure.
In general, chemical attack on thermosetting polymers could be a "go/no-go situation." With an improper setting, attack on the strengthened polyester can occur within a short time. Experts indicate that if an installation has operated with success for 12 months, it will still operate satisfactorily for a considerable amount of time. Thermosetting polymers aren't capable of handling targeted sulfuric acid (93%) and concentrated acid.
Stress corrosion is another issue to contemplate. The failure rate of glass-reinforced composites may be important. This can be especially true of composites exposed to a mix of acid and stress. Stress cracks develop once a troublesome polymer is stressed for an extended amount of time. Cracking can occur with very little elongation of the material. There is less probability of environmental stress cracking when the molecular weight of the polymer is high.
Elastomers are polymeric materials whose dimensions may be drastically modified by applying a comparatively modest force, but that will then return to their original dimensions when the force is removed. Elastomers are primarily composed of enormous molecules that tend to make spiral threads, like a coil spring, which are connected to every alternative at infrequent intervals. As a little stress is applied, these coils tend to stretch or compress, but they exert an increasing resistance as further stresses are applied.
The corrosion of an elastomer can be affected by the corrodent’s concentration and temperature. Another vital issue is the elastomer's composition. It's a standard observance within the manufacturing of elastomers to include additives into the formulation to enhance the physical and/or mechanical properties. These additives might have an adverse impact on the corrosion resistance, notably at elevated temperatures. Conversely, some manufacturers compound their stuff to boost their corrosion resistance at the expense of physical and/or mechanical properties. Therefore, it's vital to understand whether any additives are used because the corrosion resistance charts are applicable just for pure elastomers.
Elastomeric or rubber materials have a large range of applications, one of the most important being lining vessels. These are sheet-applied to a steel substrate, and are used extensively as membranes in acid-brick lined vessels to safeguard the steel shell from corrosive attack.
Chemical deterioration happens as a result of a chemical process between the elastomer and corrodent. This attack leads to swelling and a reduction in strength. The temperature and concentration of the corrodent can affect the degree of decay. Normally, a chemical attack is larger because the temperature and/or concentration of the corrodent will increase. In contrast to metals, elastomers absorb varied quantities of the substances that they come into contact with, particularly organic liquids. This may end in swelling, cracking and penetration of the substrate in an elastomer-lined vessel. Swelling will cause softening of the stuff and, during a lined vessel, introduce high stresses and failure of the bond.
Permeation is another issue that will cause lining failure. Once an elastomer exhibits a high absorption, permeation typically results. However, it's not necessary for an elastomer to have a high absorption rate for permeation to occur. Some elastomers, like the fluorocarbons, are simply penetrated, despite having low absorption. An approximation of the expected permeation and/or absorption of a substance may be supported by the absorption of water.
Permeation will create significant problems in elastomeric-lined instrumentation. Once the corrodent permeates the elastomer, it comes into contact with the metal substrate. This may result in:
- Bond failure and blistering, caused by an accumulation of fluids at the bond
- Failure of the substrate attributable to corrosive attack
- Loss of contents through the lining and substrate because of the failure of the substrate
Although elastomers may be broken by mechanical means, this isn't typically the case. Most mechanical harm happens as a result of chemical deterioration. Once the elastomer has deteriorated somewhat, the material is weakened and becomes more prone to mechanical harm from flowing or agitated media.
Some elastomeric materials are subject to degradation once placed in outdoor applications as a result of weathering. The action of daylight, ozone and chemical elements will cause surface cracking, discoloration of colored stocks, serious loss of strength, elongation and alternative rubber-like properties. Therefore, resistance to weathering should be taken into consideration.