Corrosion evaluation of inner wall surfaces of FRP tanks/pipes
There is a trend to use ultrasonic non-destructive testing to evaluate corrosion on the inner wall of FRP tanks/pipes.
I would like to discuss the corrosion evaluation of the inner wall surface of FRP tanks/pipes.
FRP is used in chemical plant equipment due to its corrosion resistance
We have introduced this in the past,
FRP has superior corrosion resistance compared to metal,
FRP is used as a structural material for facilities in chemical plants that handle electrolyte solutions and acidic and basic aqueous solutions,
FRP is sometimes used as a structural component of equipment in chemical plants that handle electrolyte solutions and acidic and basic aqueous solutions.
The main structural components are tanks and piping.
For example, in a past column, an accident involving a large amount of spilled hydrochloric acid was cited,
and examples of utilizing FRP’s chemical resistance,
I have also discussed the importance of appropriate material usage and maintenance management.
In the case of volatile hydrochloric acid,
In the case of volatile hydrochloric acid, corrosion by volatile hydrochloric acid gas is more dangerous than the hydrochloric acid solution itself,
It may come as a surprise to some of you that the corrosion caused by volatile hydrochloric acid gas is more dangerous than the hydrochloric acid solution itself.
Related Columns
Chemical-resistant FRP used for storing acidic chemicals in chemical plants (in Japanese only)
For FRP, Selection of Reinforcing Fibers Used in Layers Near Wetted Surfaces is Important
As mentioned in the reference column above,
When FRP is used as a structure that takes advantage of its chemical resistance,
When using FRP as a structure that takes advantage of its chemical resistance, it is very important to design the material composition of the wetted surfaces (or surfaces that come in contact with volatile substances of the stored chemicals).
In terms of material composition design, the following materials should be selected,
It is natural to use vinyl ester (epoxy acrylate) as the matrix resin, but the selection of reinforcing fibers is also important,
The selection of reinforcing fibers is also important.
For example, fluoric acid, which can damage glass, and surfaces where glass fibers come in contact with weak basic aqueous solutions,
It is recommended to use organic fibers, and in some cases carbon fibers, instead of GFRP.
For glass fibers, AR glass with ZrO2 or CaO added has higher base resistance,
However, leaching of Si from the fiber by basic aqueous solutions is still inevitable.
Related Columns
Alkali Resistance of Glass Fibers (in Japanese only)
The innermost layer is the harshest
FRP has a structure in which materials are stacked through a lamination process.
Therefore, when pipes and tanks are fabricated with FRP,
The innermost material is the harshest in terms of exposure to chemicals.
When evaluating the “damage risk” of piping and tanks, it is important to consider the following factors,
it is important to quickly determine the condition of the FRP near the innermost layer.
The following is a technical document that provides an overview of the above approach.
The condition of the innermost layer of FRP is being verified to see if it can be evaluated by nondestructive testing
The technical documents referred to are listed below.
You can read them online by registering.
This technical document describes the structure of tanks that store acidic and basic aqueous solutions, as well as the piping that transports them,
The inner-most layer of the structure is called the “Corrosion Barrier”, and is designed to prevent corrosion of the structure by preventing changes in the condition of the structure over time,
The term “corrosion barrier” refers to the innermost layer of a structure, such as a tank that stores acidic or basic aqueous solutions or a piping system that transports them,
We are conducting various evaluations with this in mind.
No detailed technical data is provided
At the outset, it should be noted that no detailed data is provided within this technical document,
There is not enough information to make a technical discussion.
However, we believe it is adequate to understand the outline,
Please note that we have included this information in this report because we believe it is appropriate for an overview of the data.
The author of this technical article is Geoff Clarkson, founder of a company called UTComp, Inc,
The person who wrote this technical document is Geoff Clarkson, founder of UTComp, Inc. which provides maintenance and inspection services to corporate clients using a patented technology called UltraAnalytix NDT,
I assume that this is what they have in mind.
Reference information
UltraAnalytix NDT / Nondestructive testing for FRP fitness for service evaluation
In this connection, Geoff Clarkson provides an overview of related technologies in the following video.
This video also does not go into detail.
(URL of Video is here in case that you can not watch video below.)
Below we would like to present some of the key points of this technical document.
Summary of Technical Data
The main contents are visual inspection, evaluation of chemical penetration, and validation of the application of nondestructive testing to determine the condition of the “Corrosion Barrier”, the innermost layer of a chemical-resistant FRP structural member.
Each of these is described below.
General material composition of FRP structural members with chemical resistance (corrosion resistance)
Thermosetting and thermoplastic matrix resins are introduced,
The former includes epoxy, vinyl ester, and polyester resins,
The latter include PP, PVC, PE, polyvinylidene fluoride (PVDF), etc.
Thermosetting Matrix Resins
Figure 1a shows an example of a basic stacking configuration for thermosetting resins.
Corrosion Barrier in 3 layers,
The reinforcing fibers consist of one layer of non-woven fabric (or mesh) and two layers of glass mat,
The basic thickness is 2.4mm.
The non-woven fabric is the innermost layer.
Vf is 5% in the non-woven layer and 13-16% in the glass mat layer, which is quite rich in resin.
As stated in the technical data, the stiffness of Corrosion Barrier is low due to the high resin content, and it does not seem to serve as a structural member,
It does not seem to play a role as a structural member.
Thermoplastic Matrix Resins
Figure 1b shows a similar case of thermoplastic resin.
In the thermoplastic case, the Corrosion Barrier is a plastic liner,
and no reinforcing fibers are used.
The liner is bonded (fused) to the FRP structural member.
However, there are cases where this bonding process is not used.
Corrosion Barrier thickness of 2.5mm is only one indicator
Of course, this does not mean that the Corrosion Barrier must be 2.5 mm thick,
Corrosion Barrier thickness of 2.5mm is not an indicator.
It depends on the content liquid and its concentration, the expected durability, the specification environment, and the design concept.
One Method of Material Selection
One of the most difficult issues for those who work with FRP is the selection of materials.
In the technical data presented here, ASTM Standard Practice C581 is shown as an example.
Reference
This standard is,
Standard Practice for Determining Chemical Resistance of Thermosetting Resins Used in Glass-Fiber-Reinforced Structures Intended for Liquid Service
This is a standard for evaluating the durability of thermosetting resins that require chemical resistance in wetted conditions.
Although this standard is only a “method” for material selection, it may be a useful reference.
The standard states that resins should be immersed for 1 month to a maximum of 12 months, and the following evaluations should be made.
- Changes in hardness of the resin surface
- Changes in weight and thickness of the specimen
- Changes in flexural modulus of the specimen
- Changes in hardness, weight/thickness, and flexural modulus of the specimen
It is not as simple as just copying the above in its entirety,
However, we feel that it is a good technical reference.
Next, I would like to discuss the state of the Corrosion Barrier.
Visual Inspection
An example of degradation is shown in Figure 2.
Oxidative degradation, color fading, surface delamination, and cracks are typical examples.
Cracks and surface delamination,
Focus on damage that can be visually detected,
It is easy to evaluate the deterioration near the inner surface layer,
However, it is difficult to determine the actual depth to which the deterioration caused by the chemical has penetrated,
However, it is difficult to determine how far the chemical has actually penetrated in the depth direction by visual inspection alone.
Although the visual inspection requirements for newly fabricated FRP structural members can be clarified, it is difficult to determine the extent of deterioration due to chemicals after long-term use,
What is the damage condition of FRP that has been degraded by chemicals and other agents due to long-term use?
The damage condition of FRP deteriorated by chemicals and other agents due to long-term use should differ depending on the type of chemical, time, environment of use, and material composition,
It is very difficult to determine these requirements as a unified requirement.
In any case, visual inspection alone is not enough,
In any case, it is difficult to properly determine the state of deterioration and calculate the remaining service life by visual inspection alone,
It is difficult to properly determine the state of deterioration and to calculate the remaining service life.
Evaluation of Chemical Penetration
Deterioration of FRP due to chemicals can be evaluated by the degree of penetration,
There is a way of thinking that
One example is the quantitative evaluation by elemental analysis using EDX (Energy Dispersive X-ray),
Quantitative evaluation by elemental analysis using EDX (Energy Dispersive X-ray ) is one example.
The results are shown in Figure 3.
This is an optical cross-sectional image of the Corrosion Barrier of FRP exposed to hydrochloric acid for 17 years,
The area where hydrochloric acid is assumed to have penetrated is discolored green.
Furthermore, EDX of the cross section shows that the chlorine concentration is high in the layer of the Corrosion Barrier,
The EDX image shows that the chlorine concentration is high in the layer of the Corrosion Barrier, and that it has reached the lower layer of the Corrosion Barrier.
The depth of penetration is 4mm,
Therefore, we can understand the fact that 4mm of hydrochloric acid permeates through this FRP in 17 years,
This is a fact that can be understood.
However, in order to conduct such an evaluation, it is necessary to cut out the relevant part from the structure,
This would be a difficult task for a factory that is required to operate constantly.
Validation of Non-Destructive Testing Applications
Ultrasonic testing is employed as a non-destructive testing technique.
The results are shown in Figure 4 in the Technical Data.
Figure 4b is noteworthy.
A-scope (waveform showing the amplitude from the monocular inspection) is shown.
The vertical axis is the echo height and the horizontal axis is the time.
The echo around 14μsec in the figure suggests delamination,
In terms of depth, it indicates a position of about 5 mm.
Causes of reflection echoes include,
The elastic modulus changes due to chemical damage (degradation) of the matrix resin,
The ultrasonic waves are reflected at the degraded interface.
In the text of the technical data, it is written that the modulus of elasticity of the resin has changed due to the damage, as described above,
More precisely, the damage caused the appearance of layers with different densities (porosity layers),
I think it is more accurate to say that the damage caused the appearance of layers with different densities (porosity layers).
This is because ultrasonic waves tend to reflect at boundary surfaces with different acoustic impedance.
The acoustic impedance is often expressed as Z,
and can be obtained by the product of the density ρ and the sound velocity C.
Z = ρC
Related Columns
Reflected Echoes at the Interface between Resin Deteriorated by Chemicals and Sound Resin
I would like to make a few additional observations here.
I would like to discuss the phrase “the modulus of elasticity of the resin has changed due to the damage” in the technical data.
Personally, I would like to see a strong reflection echo caused by a change in density to the extent of chemical penetration,
I personally understand that there is no change in acoustic impedance that would cause a strong reflected echo with a density change of the chemical penetration level.
Especially at the interface between reinforcing fibers and matrix resin like FRP,
Especially in the case of materials such as FRP, where there is a density difference between the reinforcing fiber and matrix resin interface, the gain and frequency are lowered,
Even if the reflection echoes mentioned above were to occur, they would not exceed the range of noise,
We believe that the above-mentioned reflection echoes, even if they occur, may not exceed the level of noise and consequently may not be detected.
Regarding the ultrasonic testing results shown as an example
One more point, I personally do not understand much about the results shown in Figure 4b.
The ultrasonic waves should be visible with the surface waves on the leftmost side and the bottom waves on the far right,
The scale and gain are adjusted so that the surface wave is seen on the leftmost side and the bottom wave on the rightmost side, and the flaw echoes that occur in between are generally seen in ultrasonic flaw detection.
However, in Figure 4b, there is an indication of “surface wave” around 19μsec,
This is because the reflected echoes at the already mentioned resin degradation interface are evaluated at the left side of the surface wave.
I have looked at the original document referenced,
The only thing it says is that a decrease in the target echo height is a guideline for degradation progression,
The details are not clear.
The reference is to Figure 7 of the following technical document.
Reference information
For Nondestructive Testing Professionals,
If you are a nondestructive testing professional and have advice on this topic, I would be grateful if you could let me know,
I would be grateful if you could advise me on this subject.
There is a correlation between the depth of chemical penetration observed visually and the location of the degraded interface depth suggested by ultrasonic testing.
Figure 6 in the technical data shows the results.
The vertical axis is the position of the degraded interface depth indicated by the ultrasonic testing described above,
The horizontal axis is the depth of the interface as revealed by visual inspection.
The two main points are as follows.
- The correlation between the ultrasonic results and the visual inspection results is steadily increasing.
- Ultrasonic results are more severe than visual inspection results (ultrasonic detection depth > visual chemical penetration depth).
This is due to the non-destructive testing called ultrasonic testing,
This is because ultrasonic testing, a nondestructive inspection method, is a very effective method to evaluate the chemical degradation of FRP structures,
This is not a major problem in the sense that the non-destructive inspection of ultrasonic testing has shown some reasonable results in evaluating the chemical degradation of FRP structures.
No correlation between bending test and chemical penetration depth
The same is shown in Figure 7 in the document.
The vertical axis is the residual flexural modulus and the horizontal axis is the chemical penetration depth.
It looks as if it is steadily decreasing,
It appears to be saturated at a certain point.
Since the detailed conditions (thickness of Corrosion Barrier, lamination composition, specimen thickness, etc.) are not described, it is difficult to make technical considerations, but I would like to consider a few things.
Corrosion barriers are NOT structural parts
Corrosion barriers are mainly made of resin and are not structural members,
Therefore, it does not contribute much to the strength or modulus of elasticity of FRP in flexural tests.
Therefore, the effect of degradation of the Corrosion Barrier on the FRP as a whole may be limited,
Therefore, the effect of Corrosion Barrier degradation on FRP as a whole may be limited to begin with.
All specimen dimensions should be the same.
Bend test results are affected by specimen dimensions.
Given this assumption, it is unclear whether this evaluation was all measured on specimens of the same thickness, width, and length.
It also appears that the dimensions of the base material from which the specimens shown in Figure 5 were cut are not the same.
More to the point, it is thought to have curvature.
Thus, testing with a specimen that has a shape factor,
In my opinion, it does not make much sense.
Multi mode tests are not recommended for FRP evaluation.
Unless it is an assumption that the geometry and material are always identical, such as in quality assurance of a product before shipment,
Bending tests are a composite mode that is not appropriate for FRP evaluation,
In other words, bending and compression are evaluated at the same time.
If you want to evaluate FRP degradation due to chemicals, etc,
shear in-plane or compression would be appropriate.
If you really want to evaluate FRP in a flexural form, then short beam is an example,
Short beam is an example.
This is not a bending test but an interlaminar shear test.
Note, however, that the modulus of elasticity cannot be obtained here.
Why is the Corrosion Barrier layer resin rich?
One additional technical point should be mentioned.
Why is the Corrosion Barrier layer rich in resin?
The penetration of not only liquids such as chemicals, but also corrosive gases and oxygen, which can also cause oxidative degradation,
In the case of FRP, it is mainly at the interface between the resin and reinforcing fibers and
In the case of FRP, it is said to penetrate into the interior (deep inside) mainly from the interface between resin and reinforcing fibers or from fiber bundles in areas not impregnated with resin.
Regardless of whether thermosetting or thermoplastic, resin layers do not have “non-uniform areas” that promote the penetration of liquids and gases as described above.
Diffusion phenomena in a pure resin layer can explain the penetration of corrosive materials such as chemicals.
This kind of material setting is also possible,
This is one of the strengths of FRP as a composite material.
SUMMARY
As a corrosion evaluation of the inner wall surface of FRP piping,
While referring to the results of visual inspection and material testing,
We introduced a case study in which the validity of the evaluation by ultrasonic testing was verified while referring to the results of visual inspections and material tests.
By applying these technologies, UTComp, Inc,
UTComp, Inc. sets inspection frequencies and life expectancies.
The details of the evaluation method are unknown, so we do not know the validity of the method,
However, it is a very important concept for the long-term use of FRP structures.
And among the nondestructive testing technologies, ultrasonic testing is also available as a handy type, etc,
Ultrasonic testing is a powerful tool for onsite evaluation.
In the case of a chemical plant, it is also possible to inspect the equipment while it is still in operation.
In the future, nondestructive testing technology,
to be applied to on-site FRP condition assessment,
may become more common than it is today.
