Metallic Corrosion Protection

The careful testing of metallic anti-corrosion coatings is essential in the marine, rail and infrastructure sectors. Tiny defects in components such as screws can have fatal consequences. FISCHER offers measurement devices with which you can test for rust protection in galvanized steel and other metals non-destructively. Mobile, easy to use and specially designed for the measurement of large objects such as bridges or scaffolding.

Metallic corrosion protection

Application notes

Duplex Measurement of Lacquer/Zn/Fe for Steel Roofing Systems from BUDMAT®

Roofs are exposed to very harsh weather conditions over many years. To avoid corrosion and guarantee the longevity of steel sheet roofing, the coating of the steel must be of the highest quality. The coating structure is typically built up of three layers of lacquer over a basic zinc coating.

The norm PN-EN 508 – 1.2010 defines the minimum requirement of 275 g/m² zinc coating for such roof coverings. On top of the galvanization there are different types of lacquers available for use, which, together, determine the coating’s final scratch resistance.

Steel sheet roofing materials coated in gloss or matte polyester typically have inner lacquer layers measuring between 27-38 µm. They are considered “thin-layered coatings” and are generally covered by a written guarantee of 10 years.

However, the most highly recommended types have “thick-layered coatings” with an inner lacquer layer 50 µm thick, which gives the steel sheets greater resistance to corrosion and mechanical damage. These high-quality products, such as those provided by the Polish manufacturer BUDMAT®, can be warranted for 30 years.

A characteristic of these sheets is their increased resistance to the kinds of frictional strain to which a roof is often subjected, such as frozen snow, or during assembly. The surface layer contains polymeror ceramic grain which prevents surface scatching.

During the manufacturing process the coated steel sheets are measured with the PHASCOPE® PMP10 DUPLEX. This instrument uses two different electro-magnetic measurement methods to determine the thickness of the paint and zinc layers in a single measurement process and displays the results separately: The magnetic induction method is used for measuring the overall thickness of the paint and zinc on top of the ferrous base material, whereas the phase-sensitive eddy current method is employed for measuring just the zinc coating, irrespective of overlying paint layers. The thickness of the paint layer is calculated automatically as the difference between these two results.

Figure 1: Principle of duplex measurement

Duplex measurements are taken from each production batch and automatically recorded from the host computer – a total quality philosophy. A quality certificate is provided with each delivery.

Because the PHASCOPE® PMP10 DUPLEX instruments are designed for mobile use, it is also possible to test the coating quality of pre-existing roofs.

Figure 2: PHASCOPE® PMP10 DUPLEX 

30-year guarantees can only be provided on steel roofing systems produced according to a high-tech, quality-based manufacturing philosophy. But that requires continuous, vigilant monitoring of the thicknesses of zinc and lacquer coatings on the steel sheet. Because of its ability to perform duplex measurements of different types of coating materials, the instrument ideally suited for this challenge is the handheld PHASCOPE® PMP10 DUPLEX. For further information please contact your local partner for FISCHER products.

Application Note prepared in cooperation with BUDMAT – one of the leading manufacturers of steel roofs and facades, gutter systems and construction profiles. For further info:

www.budmat.pl

Hot-Dip Galvanization as Corrosion Protection

For protection against the elements, exposed steel parts require an anti-corrosion coating such as hot-dip galvanization. To this end, a new directive for CE labelling of steel products and their corrosion protection will become effective in 2014. Product liability will be significantly tightened and manufacturers will be obligated to verify the thickness of the hot-dip galvanization.

Life is about to change for manufacturers of metal and steel structures. Beginning in 2014, a new CE labelling standard for steel products and their corrosion protection will shift product liability – i.e. the burden of proof for documenting coating thickness measurements – to the providers of the coating systems. For many in this field, only the most user-friendly and cost-effective measure­ment technologies will come under consideration.

Fig.1: Compact pocket-sized instruments of the MP0/MP0R family

The FISCHER MP0/MP0R product family meets precisely these needs and requirements. Due to their compact design and simple four-button handling, these instruments are flexible in on-site applications and require no costly user training. The two displays allow for easy reading in various operational positions. Visual and acoustic signals inform the user when the measurement is complete.

The hard metal probe tips are a special feature that guarantees significantly longer lifetime, even on rough surfaces. Measurement results can be easily transferred to a computer for evaluation, recording and storage using the convenient FISCHER DataCenter software.

Solutions are also available for more demanding requirements, for example the measurement of hot-dip galvanized coatings underneath a layer of paint. Specifically for this purpose, FISCHER has developed the FDX13H probe. Used in combination with the FMP instruments, it can determine the thickness of both the zinc and paint coatings in one “duplex measurement” step; the readings are displayed separately. FISCHER has thereby succeeded in simplifying for the user an extremely complicated metrology procedure, simul­taneously presenting the results of two different physical measurement principles in one easy operation.

Whether the compact and cost-effective MP0/MP0R gauges or the powerful FMP models with exchange­able probes, FISCHER has the right high-precision instrument for determining the thickness of hot-dip galvanized coatings. Your local FISCHER repre­sentative will be happy to answer any questions you may have.

Measuring the zinc plating on nails with PHASCOPE® PMP10

Nails and other fasteners are coated to protect them against rust; the thickness of the zinc layer corresponds directly to how long they can resist corrosion. Extended warranties and constantly-changing regulations make it more important than ever for the fastener industry to assess whether the quality of the plating meets relevant standards.

A wide range of fastener products (e.g. nails, screws, bolts, etc.) that are zinc plated, require testing to specification. Typically, this would be on hemispherical surfaces, threaded stems or cut ends. For non-destructive testing, one can employ either high-precision x-ray fluorescence (XRF) or the more cost-effective phase-sensitive eddy current method. This method fulfills the requirements of the EN (DIN) 14592 classification. It is better suited than other electro-magnetic methods because of its ability to measure electrically conductive coatings – even on rough surfaces – on a variety of substrates. It also offers great advantages in measuring small objects since the geometry of the part being measured exerts very little influence on the measurement itself.

Optimal measurement results require a special probe that enables small measurement areas on curved, rough surfaces. The handheld PHASCOPE® PMP10 instrument, used together with an ESD2.4 probe, is the ideal combination for this measurement task.

The following table compares the average values of the zinc thickness as measured with a PHASCOPE® PMP10 to measurements taken with an XRF system.

AREA

A

B

C

PMP10

15.5

8.7

15.2

XRF

15.0

8.3

15.0

Tab.1: Comparison between coating thickness measurements taken by PHASCOPE® PMP10 and x-ray fluorescence

Besides the coating thicknesses, the values also show the distribution of the zinc layer over the nails, indicating the quality of the plating. The results further illustrate how well the PHASCOPE® PMP10 measurements correlate with those taken using x-ray fluorescence.

The handheld PHASCOPE® PMP10 and the probe ESD2.4 together form an affordable measuring system that provides good precision and accuracy in a portable unit for quick and reliable testing of zinc-coated fasteners. For further information please contact your local FISCHER representative.

Material analysis on corrosion-resistant fasteners for longevity

Screws, nuts, washers and bolts can be found everywhere holding our world together, but some of them lead a much harder life than others, especially those used in harsh environments like offshore installations. Because nobody ever wants to dismantle a wind turbine or oil rig just to replace the fasteners, special corrosion-resistant alloys of stainless steel are used for them. This enables the supplier to guarantee a much longer service life, but that, in turn, requires clear and rigorous supervision of the material composition.

Any part of a marine structure – whether wind turbines, oil rigs or ships – that is permanently exposed to salt water and inclement weather must be shielded against corrosion. The same precautions taken for protecting the surfaces of these structures must also apply to even the smallest parts holding them together. Every offshore installation contains many thousands of fasteners – and every single nut, bolt or screw has an important job to fulfil, for at least as long as the structure itself.

Fig.1: Oil rig and other typical marine structures in an offshore setting

To avoid too-frequent maintenance or, even worse, replacement of all the bolts before the structure itself is worn out, the fasteners are made of special corrosion-resistant materials that guarantee a much longer lifetime than usual. Different kinds of stainless steel, with cryptic names like “316”, “317” or “A4”, are used, each with its own alloy composition. For example, A4 stainless steel contains molybdenum, which significantly increases both strength and corrosion resistance to withstand caustic agents such as salt water (chlorides) and acids. It is rec­ommended for use in highly aggressive environments.

Fig.2: Variety of nuts and bolts

A good way to ensure that these components are made of the correct material is to analyse its exact composition. This is where the non-destructive x-ray fluorescence method (XRF) is ideal. Using the FISCHERSCOPE® X-RAY XDLM® with its powerful WinFTM® software, it is both fast and easy to take very precise and accurate measurements of the alloy, in order to determine its composition. And using a small collimator together with a microfocus x-ray tube allows measurement even on tiny structures.

Fig.3: WinFTM® diagrams of an X-Ray measurement on a bolt, searching for molybdenum. The left diagram shows a “normal” stainless steel bolt, the right one a bolt containing molybdenum for longer life.

To verify if fasteners have the correct alloy composition needed for longevity under offshore and other harsh environments, the FISCHERSCOPE® X-RAY XDLM® is the best choice. For more information, please contact your local FISCHER representative.