Surface Inspection

For the surface testing of large surfaces in marine, rail and infrastructure sectors, mobile measurement devices from FISCHER deliver reliable and non-destructive results. Whether for controlling the redensification of anodized aluminum or the examination of coatings for hairline cracks and pores: we offer numerous customized measurement solutions for the testing of various surfaces.

Surface inspection

Application notes

Thickness measurement of INCONEL® coatings on waste heat recovery boilers

Known for their resistance to oxidation and corrosion, Inconel alloys are often used in extreme environments subject to high heat and pressure. This is why Inconel coatings are often used in waste heat recovery boilers, which recycle the energy contained in hot exhausts given off by various manufacturing processes. However, to assure the functionality of the coating, a minimum thickness must be guaranteed, and thus, measured precisely.

Energy that would otherwise be lost in discharged exhaust is captured and reclaimed in waste heat recovery boilers. The hot flue gas is fed into the boiler where it heats up water flowing through pipes within the housing, producing either hot water for use in other industrial purposes or, depending on the temperatures involved, steam that powers a gas turbine to generate electricity.

To protect them against these often aggressively corrosive gases, the pipes are coated with a 2 mm thick layer of Inconel material. However, once this protective coating wears down to less than 1 mm, the pipe bundles need to be retrofitted. This is why monitoring the thickness of the Inconel coating is so important during maintenance of waste heat recovery boilers.

The DELTASCOPE® FMP coating thickness measuring gauges rise to the challenge of this application. The external FGB2 probe allows free positioning, enabling measurements from any angle on the pipe bundles. The measuring range for this probe is 0-5 mm – with an accuracy and precision of less than 1.5% for readings on coatings between 0.1 and 3 mm thickness. The matrix mode of the FMP software delivers a nice overview of the coating thickness distribution over the various pipe bundles. Measurement results can be easily transferred to a computer for evaluation, recording and storage using the convenient FISCHER DataCenter software.

Fig. 1: DELTASCOPE® FMP Family.

To avoid the costly – if not disastrous! – consequences of corrosion damage on pipe bundles in waste heat recovery boilers, the thickness of the Inconel coating must be inspected regularly. The DELTASCOPE® FMP instrument with the FGB2 probe makes this task easy. For further information please contact your local partner for FISCHER products.

Measurement of Thick Coatings on Pipelines

On oil pipelines, propylene coatings serve a multitude of important purposes including corrosion prevention and insulation, but they are expensive. In order to ensure appropriate thickness for guaranteeing performance without wasting valuable material, the application process needs to be controlled carefully.

In the oil and gas industry, it is common to transport the liquid or gaseous goods through undersea pipelines. Insulation is not only necessary to avoid thermal losses, since the oil is mixed with hot steam to improve its fluid properties, but also to protect the pipe from the extreme temperatures (e.g. in the polar regions), high pressure and corrosive waters found at the bottom of the ocean: Any penetration of the coatings can eventually result in leakage and environmental disaster.

Therefore, for both corrosion prevention and insulation, the pipes are typically enclosed in one or more layers (often up to 100 mm thick in total) of polypropylene, a highly resilient thermoplastic polymer that can withstand the harsh deep-sea conditions. To ensure the layers are properly applied – sufficiently thick but without wastage or delamination – rigorous quality inspections must be performed using a highly accurate instrument that can measure coatings of such dimensions.

Figure 1: Coating thickness measurement on a pipeline segment with the FA100 probe connected to a DUALSCOPE® FMP100

Especially for these demanding requirements, FISCHER has developed the FA100 probe, which fully covers the thickness range of up to 100 mm. The FA100 can be connected to the handheld instruments of the FMP family, allowing mobile use wherever needed. The handy FMP gauges are available with either a touchscreen or – even more robust – conventional buttons.

Compared to ultrasonic instruments, the FA100 plus FMP combination provides more accurate results and easily handles multi-layer structures without negative influence, irrespective of coating material type. The results in Table 1 below demonstrate the consistency of 26 readings taken on a segment of insulated pipeline.

Mean Value

41.29 mm

Standard Deviation

0.599 mm

Coefficient of Variation

1.5 %

Range

2.35 mm

Minimum

40.5 mm

Maximum

42.9 mm

No. of Readings

26

No. of Blocks

5

Table 1: Results from pipeline inspection with probe FA100

Measuring the coating thickness is as simple as sliding the FA100 probe lengthways or sideways over the sample surface. In automatic measurement mode, the gauge screen shows a graphic representation of the coating thickness, which helps the user to assess coatings for evenness (concentricity/eccentricity). Final results are written to PDF via the powerful DataCenter software. Measurement area pictures and thickness annotations can be stored in application memories assigned to job sections, shifts or operators.

With the easy-to-operate handheld instruments of the FMP family, used in combination with FISCHER’s special probe for measuring thick layers, the FA100, pipeline coatings can be assessed precisely to ensure their quality and performance. For further information please contact your local FISCHER representative. 

Porosity testing on pipelines and offshore structures

Ensuring the long-term protection of parts exposed to the extremely harsh conditions found in offshore environments requires intact anti-corrosion coatings. Any void, gap or pore in the coating can significantly shorten the lifetime of protected components. To control the quality of these crucial coatings, porosity testing is mandatory.

Basically all offshore structures – such as ships, oil rigs, cranes, containers and pipelines (including fittings, valves, etc.) – are surface coated to shield them from the harsh environmental conditions found at sea. Because even a tiny pinhole can spoil the protective function, it is necessary to rigorously inspect the coating for integrity. But the most careful visual observation still cannot detect all the pores, cracks and thin spots (less than specified thickness) that can form during the coating process. High-voltage porosity testing is the only truly reliable way to inspect the corrosion-protection coatings on all kinds of offshore structures; the device commonly used for this is often called a “holiday detector”.

Fig.1: Virtually all metal structures in offshore use, such as ships, cranes and containers, wear a protective coating that needs to be inspected to ensure that it can withstand the extreme conditions

The test method is based on the fact that all electrically insulating coating materials have a much higher disruptive strength than air does. High voltage is applied using for example a brush-like electrode which is moved across the surface of the specimen. In the case of a defect (pore, scratch, etc.), a spark-over occurs, which is indicated acoustically and optically by the system.

FISCHER’s new POROSCOPE® instrument is specifi­cally designed to fulfil the requirements for coating inspection in offshore environments. The measurement head HV40 (with a voltage range of 8-40 kV) even allows for the testing of thick coatings.

Fig.2: Oil platforms require robust protection against the elements

The portable HV 40 is a sturdy, metal-clad instrument designed for practical application on rugged jobsites. The high voltage is generated inside the probe head, improving both operator safety and ease of use: it eliminates the need to drag long, bulky HV-insulated cables across wet ship decks and tanks – which also makes the instrument far less sensitive to moisture.

Fig.3: Schematic of how the POROSCOPE® works

The intactness of the corrosion protection on any metal structure in an offshore environment is critical to its performance and longevity. The new FISCHER POROSCOPE® HV40 is perfectly suited for per-forming the porosity testing required in this field. For further information please contact your local partner for FISCHER products.

Weather resistance of sealed anodized coatings

Aluminum is often used for architecturally pleasing building façades and other structures exposed to weathering influences. Lightweight and fairly easy to work, it is also durable and requires little to no maintenance over the long term when effectively protected by an anodized coating. But to ensure the reliability of the corrosion protection, the sealing of the coating must be verified.

Pure aluminum is actually highly susceptible to corrosion. However, in contrast to iron, it naturally forms a corrosion-inhibiting layer on its surface when it comes into contact with oxygen, which preserves it very well under normal environmental conditions: When this protective oxide layer is scratched away it is quickly replenished upon renewed exposure to oxygen. Nonetheless, for many applications, especially out-of-doors, this natural protection does not suffice because even the smallest entrapment of heavy metals can prevent the formation of a seamless oxide layer, allowing corrosion to get in and cause damage.

In order to improve on the natural oxide film, an electrolytic passivation process called anodic oxidation, or anodising, is often used. In an acid bath, oxygen released by an electrical current immediately reacts with the aluminum. It forms two oxide layers: a very thin insulating film (barrier layer) directly on the metal and, above that, a thicker, porous layer. Due to its microporous structure, the outer layer is vulnerable to contamination, requiring a post-treatment (hydration) that expands the coating and causes the pore walls to swell and close off at the surface (sealing).

Fig.1: Schematic diagram of the anodized film on aluminum before (left) and after (right) the sealing process

Although the ultimate quality of the sealing depends on a variety of factors including the condition of the oxide layer, the hydration time, the sealing bath temperature, the pH value of the solution, etc., it can be examined simply and non-destructively by measuring the electrical admittance (Y). Using FISCHER’s handheld ANOTEST® YMP30-S instrument, the coating’s admittance can be determined in situ according to standards such as DIN EN ISO12373-5 and ASTM B 457-67. An electronic reference is also available for cross-verification of the instrument.

Fig.2: Testing the sealing of an anodized façade with ANOTEST® YMP30-S

With the ANOTEST® YMP30-S, the sealing of anodized aluminum can be tested by measuring its electrical admittance. This makes it possible to determine the corrosion resistance of façades and other aluminum components exposed to weathering influences. For further information please contact your local FISCHER representative.

Complying with regulations concerning coating thickness inside storage tanks

Anticorrosion coatings on the interior of storage tanks are critical when storing many types of products, whether saltwater or freshwater, gasoline, ballast, foamy liquids, or diesel. Typically, regulations are in place to ensure that the right coating is used. Quality inspection of the coating is an essential part of compliance with the regulations.

Although there are many such regulations, the Petrobras standard, N-1201:2008 – “Anticorrosion Coatings for the Interior of Storage Tanks”, is a good example. A multinational company headquartered in Brazil, Petrobras uses the above-mentioned standard wherever it has operations. This standard is in the public domain and can be accessed online from their database; it regulates all painting processes applied to storage tanks, from surface preparation, to painting and coating, through to final inspection.

Item

Tank type

Stored product

Region to be painted

Paint type

1

Fixed roof tank

Saltwater
Freshwater -

potable or not

Gasoline
Ballast
Foamy liquids
Diesel
Aviation fuel

Inside surface

Coating Type 1

6

Fixed or floating roof, horizontal or underground

Ethyl alcohol

Inside surface

Coating Type 2

7

Fixed or floating roof, horizontal or underground

Crude Oil

Formation Water

Inside surface

Coating Type 3

Tab.1: Anticorrosion coatings used in storage tanks according to the Petrobras standard N-1201:2008 (unofficial translation)

The Petrobras standard N-1201:2008 prescribes the following coating types and coating thicknesses for the different products to be stored, as listed in Table 1:

4.2.1 Coating Type 1

4.2.1.1   Primer: one layer of epoxy-zinc phosphate paint: 100 µm

4.2.1.2   Top protective coating: two layers of non-solvent epoxy paint: 150 µm minimum thickness per layer.

4.2.2 Coating Type 2: one layer of zinc-ethyl-silicate paint: 75 µm

4.2.3 Coating Type 3: one layer of non-solvent epoxy resin, cured with polyamine and embedded with ceramic or fiberglass: 800 µm

Once they are applied, the coatings need to be inspected – a tricky job, because this is done in place! The enormous tank still smells sharply of noxious paint fumes as a technician wearing bulky protective gear and a headlamp (perhaps the only source of light) is lowered into the slippery vessel to measure the coating thickness at a number of positions. And, since a normal handheld gauge with integrated probe will not work, as the tank geometry does not permit it in some spots, an external probe is necessary.

FISCHER developed the MP0x-FP products for just such applications performed under difficult conditions. While other thickness gauges on the market require the use of both hands (one to hold the instrument and the other for the probe), the wristband accessory is indispensable when the task itself requires some juggling.

Fig.1: Single-handed measurements with the MP0R-FP. The display is visible like a watch.

Another very practical advantage of the MP0x-FP series is that calibration can be done outside the tank using FISCHER calibration accessories to maximise accuracy. This procedure only needs a piece of the original tank material, thus to avoid that the entire calibration procedure has to be performed directly in the tank.

Besides the unmatched accuracy and repeatability performance of the MP0x-FP series from FISCHER, extra features like the wristband or the easy calibration process make these coating thickness gauges the ideal choice for checking compliance with regulations like the Petrobras standard N-1201:2008 for storage tanks. For more information about the MP0x-FP series, please contact your local FISCHER representative.

Coating Thickness Measurements on Coated Aluminium Blinds

To protect them against harsh weather conditions, exposed parts require a paint, lacquer or anodised coating of a certain thickness. Accurately inspecting the coating thickness is a challenge for the practitioner, espe­cially on curved surfaces, because the geometry of the sample influences the measurement result.

Measuring on coated aluminium blinds clearly illustrates a quandary that arises in common practice: The coating thickness should be measured on both the convex and concave sides. While aluminium as substrate material calls for the eddy current principle, precisely this method is extremely sensitive to variant geometries: What is actually being measured here are the changes, induced by the thickness of the coating, in the alternating electro-magnetic field generated by the probe. These changes, in turn, depend strongly on the sample shape. Meas­urement results for coatings on convex substrates are inflated, whereas on concave surfaces the thickness is under­estimated.

Fig.1: Measurement of the coating thickness on blinds using the DUALSCOPE® FMP100 with the FTD3.3 probe

For accurate measurement with a conventional probe, a calibration is required for every radius of curvature on a bare original part: in this case, on an uncoated blind. Specifically for such applications, FISCHER has devel­oped the curvature-compensated probe FTD3.3. With this probe, calibration on a flat metal sheet suffices – without any risk that the actual curvature will falsify the measurement result. Since FISCHER eddy current probes also compensate for conductivity, the metal sheet can even be made of a different aluminium alloy.

If one calibrates both a conventional probe and the FISCHER FTD3.3 probe on a flat metal sheet and then takes comparison measurements on various radii, it is possible to see just how strongly curvature influences the measurement results (see Figure 2). While with the conventional probe, the coating thickness seems to increase disproportionately with increasing curvature, it is measured correctly with the curvature-compensated FTD3.3 probe.

Fig. 2: Comparison measurement: conventional probe versus curvature compensated probe FTD3.3

The curvature-compensated eddy current FTD3.3 probe and the handheld instruments from FISCHER make it easy to accurately measure paint, lacquer and anodised coatings on any part geometries – without additional calibration. This means much faster and simpler measurement preparation than with conventional probes. For further information, please contact your local FISCHER representative.

Microhardness measurements of paint coatings shorten weathering tests

Paint for architectural coatings is not only used to give surfaces an attractive appearance, but also plays a very important role in protecting facades against external damage and corrosion. To avoid waiting years to see if the coating really protects the surface, simulating and measuring weathering influences is necessary.

Paint coating systems are exposed to severe environmental influences like strong temperature variations, moisture and aggressive media such as acid rain, insect residue or strong cleaning agents. Facade coatings should withstand such influences and have quality characteristics such as light fastness, weathering resistance and easy cleaning.

The characteristics of such coatings depend not only on the thickness, but also on hardness, elasticity, degree of polymerisation and resistance to UV radiation. These parameters can be determined using the instrumented indentation test.

To demonstrate weathering influences, measurements were performed on samples with original surfaces (reference), on samples after 400 hours of QUV radiation (equipment weathering) and after 1 year Florida exposure test (outdoor weathering).

Fig. 1: Influence of weathering on the Martens Hardness of polyester powder coating.

The reference sample (green plot) without weathering does not show a hardness increase at the surface. The sample exposed to weathering outdoors for 1 year in Florida shows a slight increase of hardness near the surface. The sample exposed to QUV irradiation for 400 hours shows the largest hardness gradients. Reason therefore is a change in the molecular structure of the paint. Cross-linking of the paint molecules lead to an increase in hardness caused by the repeated alternation of drying, moistening and irradiation. As outdoor weathering often spans a number of years and involves very expensive sample holders and large standing areas, artificial weathering is used to simulate such outdoor weathering.

With the FISCHERSCOPE® HM2000 hardness meas­uring instrument, the effects of weathering tests can be measured easily and accurately, therefore saving costs and shortening time compared to outdoor testing significantly. Ask your local FISCHER repre­sentative for further information.