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Galvanized Steel Testing and Failure Analysis
- Failure Analysis of Painted Galvanized Steel
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- Engineering of Finishing Plant
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- Failure Analysis of Hot Dipped Galvanized Materials
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- Failure Analysis of Electroplated Components
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- Failure Analysis of Painted Galvanized steel
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- Failure Analysis of Continuous Galvanized Strip
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- Failure Analysis of Galvanizing Application Processes
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- Failure Analysis of In-Service Electroplated Products
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- Chromate Conversion Coatings
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- Failure Analysis of Powder Coatings
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- Galvanized Substrates and Preheat Process
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- Chemical Analysis of Plating Solutions
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- Surface Preparation of for Coating Application
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- Plating Procedures and FA
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- Chemical Analysis of Electroplating Layer(s)
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- Welding, Soldering, and Brazing of Electroplated components
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- Accelerated Environmental Testing of Electroplated Materials
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- Electrochemical Corrosion Testing
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- Metallurical Investigations
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- Micro-Hardness and Adhesion Testing
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- Salt Spray, QUV, and Humidity Testing
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Matco Services, Inc. has the experience needed to solve the most difficult problems in hot dip galvanizing, electrogalvanizing, painted galvanized, and numerous other metallic surface treatments. We have experience in the application process, from electro-coating, hot dip, and metallizing, to mechanical plating, barrel plating, electroless plating, chemical conversion coatings, and zinc-rich paints. Our team has the knowledge and experience need to solve all of your galvanizing and metallic plating and deposition processes.
Among the technical areas in galvanizing and other metallic deposition systems for which our team can provide valuable assistance include metallurgical evaluation, surface analysis, finishing operations, painting specifications, coloring, life expectancy determination, process control, quality control, quality assurance, and research and development. In addition to metallic zinc systems such as galvanized and galvanneal, our team has extensive experience in tin and tin-zinc, aluminum and zinc-aluminum, nickel, chromium, and other systems.
In addition to acquiring an in-depth understanding of the processes of galvanizing and other metallic deposition as well as their key controlling parameters, our team has studied numerous types of failures in the field and in the lab. These failures have ranged from Liquid Metal embrittlement, cracking and improper surface preparation prior to application, to contamination and improper process conditions and compositions, to failures due to improper field installation or application.
  
Out gassing defect (pin holes) in painted electro-galvanized steel
The Matco team has the technical expertise, analytical instrumentation, and equipment to solve your material failures using galvanized and other metallic deposition systems. Dr. Zee, Matco’s senior scientist and corrosion/materials engineer, has over 25 patents in zinc coatings. At your request training seminar on the methods used to test, inspect, evaluate and assign a ranking to describe the condition of aged (weathered) galvanized coatings on various substrate types will be presented. The training seminar is geared towards entry level corrosion inspectors and technicians.
As an additional resource for those that have a comprehensive understanding of this material, a training course specific to the failure mechanisms commonly observed in the field in addition to the methods used to reduce or eliminate these forms of failure can also be provided that takes into consideration the specifics of your manufacturing process.
Not only do we understand the plating process, but we have also seen the many different types of plating failures in the field. From improper surface preparation before the application, to the contamination of the bath causing zinc adhesion failure. The Matco team has the equipment and expertise to solve your galvanized material failures.
Painted Galvanized Steel - Common Concerns
Painted galvanized steel has a variety of issues that must taken into consideration and accounted for when a failure occurs. In the following section a review of galvanized steel and painted galvanized steel processes is provided, as well as the fundamentals and requirments for painted galvanized steel products. The corrosion processes found with both galvanized steel and painted galvanized steels are discusses. In the evaluation of painted galvanized steels, various laboratory test techniques have proven most useful, with electrochemcial impedance spectroscopy (EIS) as a sensitive quantitave technique for measureing coating degradation and corrosion protection. Case studies that detail the lab testing of replacement and repair coatings for galvanized steel with protective paint coatings that have failed are summarized.
In general, galvanizing is the deposition of zinc onto a metal surface, usually steel, which creates a sacrificial layer designed to inhibit corrosion of the metal being coated. Zinc is a highly reactive metal which reacts readily into hydroxides and carbonates on its surface. This reaction occurs on the galvanized surface before the corrosion of steel substrate takes place.
Painted galvanized steel has been in commercial use since early 20th century. The main attributes that makes it so desirable in architectural applications are:
1) Corrosion resistance and galvanic protection
2) Color retention
3) Formability
4) Cost
The painted galvanized steel is used as architectural panels in roofing and siding, utility poles and lattices in electric transmission lines and when e-coated as automobile body panels and various components. The painted galvanized steel finds its way to numerous applications that require galvanic protection due to mechanical damage. The latter makes it the most desirable coated material in atmospheric and some soil water immersion applications.
Surface Preparation of Galvanized Steel Prior to Painting
Knowledge of the condition of the galvanized surface layer before paint application is critical to producing an effective coating system. There are three general conditions experienced on the galvanized surface layer.
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Freshly galvanized – this condition usually is seen only within the first 48 hours after galvanizing. In the freshly galvanized condition, there are little or no zinc hydroxides or carbonates on the surface.
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Partially weathered – this condition is usually seen from between 48 hours to 2 years of outdoor exposure of the galvanized surface. The surface possesses a thin layer of zinc hydroxides and/or carbonates (patina) on the surface which is not well adhered to the substrate.
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Fully weathered – this condition is typically seen after 2 years of exposure of the galvanized surface, and possesses a thick adherent patina layer on the surface.
To obtain good adhesion of a paint coating, the galvanized surface should be rather flat with no protrusions and slightly roughened and profiled to provide a larger surface area of adhesion. Care must be taken not to damage or remove the galvanized coating.
One of the ways to increase the surface area is with sweep blasting. Sweep blasting creates a surface profile for the coating to create tighter adhesion of the coating to the galvanizing, and is suitable for both freshly galvanized and partially weathered galvanizing. However, for partially weather galvanizing, it may be advisable to provide chemical surface treatments to remove any possible contaminants and improve the adhesion of the coating
In sweep blasting, care must be taken not to remove too much of the zinc coating. The particle size of the blasting medium should range between 200 and 500 microns. The types of blasting mediums used successfully have been aluminum/magnesium silicate and organic materials such as corn cobs, walnut shells, limestone, and mineral sands with a hardness of 5 or less. The blast pressure should be 40 psi maximum with a blasting angle of 45 degrees at a distance from 300-400 mm. Finally, the nozzle type should be a minimum 10 mm venturi.
All these parameters will cause minimum damage to the galvanized coating and will remove no more than 10 microns of zinc from the surface. It is also important to ensure that the air used in the blasting is dried air and not saturated with water. Moisture added to the zinc patina will form carbon dioxide and affect the adhesive ability of the substrate.
Fully weathered galvanizing does not require a sweep or brush blast because the tightly adherent zinc patina has an excellent surface profile and is tightly adhered to the substrate. However, a high pressure power wash is required to remove contaminants.
Sometimes during handling or installation, the galvanized steel may get damaged. These areas should be repaired by zinc-rich coatings first before coating the structure.
The sweep blast procedure is the most difficult part of the process of coating galvanized steels, as it is prone to several potential operator errors. These errors include the following.
- Removal of too much zinc, wasting zinc from the galvanized layer.
- No profile, leaving residual contaminants and lack of adhesion of the paint coating.
- Cracking of the galvanized layer, leading to blistering and peeling of paint coating later or in the service conditions
There are three primary causes of failures of coated galvanized steels that result from improper surface preparation of the galvanized surface prior to coating. These are over-blasting, under-blasting, and residual surface contamination.
Over-blasting involves the removal of too much zinc during the blast procedure. This results in areas of little or no zinc remaining on the surface, and cracks the zinc layer and results in its delamination. Under-blasting is the removal of too little of the zinc patina during the blast procedure. This leaves contaminants on the surface, which leads to premature coating failure. Residual surface contamination, which may be picked up even after a successful blasting operation, is also responsible for poor coating adhesion and premature coating failure.
Various chemical treatments may be employed prior to paint and organic coating application to the galvanized surface. Zinc phosphate treatment provides a good surface for the coating to adhere to. However, zinc phosphate is not recommended if a zinc-rich primer is to be applied in cases where galvanic protection is required. So-called wash primers are used to neutralize hydroxides and carbonates on the galvanized surface, and they promote coating adherence.
Finally, after all the necessary steps have been taken for good surface preparation, the coating itself should be compatible with the galvanized coating to create a successful synergistic effect. Usually a two coat system is used. The first coat is fully compatible with the zinc surface and is considered a "tie coat". The second coat is the top coat to protect the "tie coat."
When coatings are applied to new structures or where the previous coating has been completely removed, the following coating systems with adequate thickness are acceptable in highly corrosive environments. Note that the best top coat is aliphatic polyurethane. However, these types of coatings would cost too much. Using a thin layer of aliphatic polyurethane with the proper primer and intermediate coat would be sufficient if the cosmetic appearance of the final product is not an issue.
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System
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Primer
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Intermediate Coat
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Top Coat
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1
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Inorganic Zinc Rich
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Epoxy
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Aliphatic polyurethane
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2
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Waterbased inorganic zinc rich
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Waterbased Acrylic
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Waterbased Acrylic
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3
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Polyurethane organic zinc rich
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Polyurethane
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Aliphatic polyurethane
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4
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Epoxy organic zinc rich
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Epoxy
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Aliphatic polyurethane
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5
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Polyurethane Micaceous Iron Oxide
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Polyurethane Micaceous Iron Oxide
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Aliphatic polyurethane
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6
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Polyurethane Organic Zinc Rich
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---
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High Build Polyurethane
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7
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Epoxy organic zinc rich
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Waterbased Acrylic
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Waterbased Acrylic
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8
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Thermally sprayed zinc
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---
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Waterbased Acrylic
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9
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Thermally sprayed zinc
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When the coatings are involved in spot repairs and/or overcoatings, the following coatings are suggested over the previous polyurethane layer in a highly corrosive environment:
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System
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Primer
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Intermediate Coat
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Top Coat
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1
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Epoxy
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Epoxy
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Aliphatic Polyurethane
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2
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Polyurethane
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Polyurethane
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Aliphatic Polyurethane
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3
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Epoxy Mastic
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Epoxy Mastic
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Aliphatic Polyurethane
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4
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Water based Acrylic
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Water based Acrylic
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Water based Acrylic
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Using a coating over galvanized steel is called duplex coating. A zinc-rich layer and coating in a synergistic combination produce a corrosion protection approximately 2 times the sum of the corrosion protection that each alone would provide. Additionally, duplex coatings are easy to repaint, are considered excellent safety marking systems, and have good color-coding. Painting over galvanized steel that has been in service for many years also extends the life of the zinc coating.
Types of Paints/Coatings That Have Worked With Galvanized Steels
Factory applied coatings include baked on powder paint, sprayed on chemically cured epoxy resins, electrostatically applied fusion bond epoxy and sprayed on coal tar or petroleum based enamel. Field applied coatings are usually brush applied enamels, mastics, polyurethane, reinforced plastics or epoxies.
Zinc-rich paints - Zinc-rich paints have been known for their excellent paint adherence to both new and weathered galvanized surfaces. They have been used in US for more than 75 years due to their barrier and cathodic protection. In many studies, zinc-rich paints outperformed all other classes of paint. One of the main reasons for its success is that it has the same characteristics as the galvanized zinc coating. These paints synergistically combine with the desired properties of the metallic zinc coating. In addition, zinc-rich paints are widely used for repairing damaged galvanized coatings. These types of paints are often used alone but for a more attractive finish, a top coat is used.
Aliphatic polyurethane - Aliphatic polyurethanehis is a two component system, generally applied over a polyamide epoxy primer. Aliphatic polyurethanes have superior weathering and chemical resistance with good adhesion and an enamel-like finish. This type of coating requires stringent application procedures. The cost per gallon precludes the development and use of aliphatic polyurethanes for protective coating applications. However, in urethane films, excessive intra-film bubbling may occur due to carbon dioxide formation or air entrapment during fast curing.
Epoxy-polyamide cured - This type of epoxy would have excellent adhesion to any type of galvanized surface. However, since they are aromatic, they are not resistant to sunlight. Many times epoxy-polyamide cured coatings are used as the ‘tie coat’ and then an aliphatic polyurethane top coat is used. This type of system is considered to be a superior combination for barrier protection coating.
Latex acrylics - Latex acrylics are fast drying, water-based materials that have excellent adhesion, durability, and weathering characteristics. Usually this type of coating is used as a top coat. Not only is this type of coating suitable for new and weathered galvanized steels, it is environmentally friendly.
Corrosion Resistance of Painted Galvanized Steel
The corrosion resistance depends on corrosivity of the environment and the type of exposure. In general, corrosion resistance depends on the following four factors:
1) Atmospheric exposures/UV.
2) Underground soil water table exposure.
3) Coating adhesion
4) Presence of thinly coated or uncoated spots.
Atmospheric exposures do not require thick coatings. However, the underground soil and water exposures demand much more thickness. In general low ph and acidic chloride environments will require pin hole free corrosion resistant coatings with no mechanical damage.
Laboratory Testing and Failure Analysis of Coating on Galvanized Steel
There is a wide variety of testing methods currently available for evaluation of paints and coatings on galvanized steel. Sophisticated and highly calibrated laboratory equipment can detect the slightest imperfections on a specimen, and accurately identify the inherent characteristics.
A macroscopic examination of the surface of the selected specimen begins this stage of analysis, followed by a microscopic examination. A close examination using a stereo microscope at magnification of 50x or less may reveal that one of the layers is brittle and full of cracks, or perhaps that an entire layer of paint is missing. An examination of failed and non-failed samples may reveal that all of the failed samples are of improper thickness. Optical and electron microscopy at magnifications ranging from 50x to 1000x magnification can be used to examine the cross section of coated galvanized samples for voids or inclusion, as well as observation of underlying corrosion products on galvanized substrates.
Physical testing provides important characteristics in the evaluation of a coated galvanized specimen which may reveal primary causes for degradation and failure. Important physical tests include thickness testing, pin hole testing, adhesion testing, determination of the plane of delamination, hardness testing, and surface roughness (profile) testing. Pin holes are caused by poor application technique, solvent evolution from the film, corrosion due to trapped materials, and the presence of sharp globules on the surface which are difficult to completely coat initially and easy to abrade subsequently. Poor adhesion is caused by improper surface preparation procedure, as well as incompatibility of coating layers or of the primer with the substrate. The determination of the plane of delamination of a failed coating is critical to ascertaining the possibility of coating layer incompatibility or improper surface preparation.
A chemical analysis of the paint or coating, as well as the galvanized substrate and corrosion products is usually the next step. Chemical analysis techniques typically used in the laboratory for paint and coating failure analysis are Fourier transform infrared spectroscopy (FTIR) for organic functional group analysis, gas chromatography – mass spectrometry (GM-MS) for organic compound separation, identification and quantification, differential scanning calorimetry (DSC) for melt range and thermal properties, scanning electron microscopy (SEM) with associated energy dispersive x-ray spectroscopy (EDS) for elemental analysis, and Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) for surface elemental analysis.
Accelerated environmental exposure tests, such as salt spray (fog) tests, humidity tests, and ultraviolet light (QUV) exposure tests can help to confirm the proposed failure mechanism of a painted or coated galvanized sample. Accelerated exposure testing can be complemented with electrochemical impedance spectroscopy (EIS). The organic coating resistance generally degrades with time. The degradation is associated with corrosive ions and water penetration into the coating, transport of ions through the coating, and subsequent corrosion reactions at the polymer–metal interface. Typical standard coating immersion tests can take hundreds or thousands of hours. However the EIS technique can provide reliable data on performance in rather short time.
Electrochemical Impedance Spectroscopy
The resistance of an organic coating generally degrades with time. The degradation is associated with corrosive ions and water penetration into the coating, transport of ions, through the coating, and subsequent corrosion reactions at the polymer–metal interface. Typical standard coating immersion tests can take hundreds or thousands of hours. However the electrochemical impedance spectroscopy (EIS) technique can provide reliable quantitative data on performance in rather short time.
The electrical impedance of a coating is the most direct measure of a coating’s barrier properties and ability to protect against corrosion. All coating properties relevant to corrosion protection can be described in terms of their effect on coating impedance: Embrittlement, cracking, excessive porosity, poor adhesion, and ultraviolet degradation are common coating problems whose ultimate failure mechanism is the creation or enlargement of channels through which water, dissolved oxygen, and soluble ions can reach the substrate. Electrochemical Impedance coupled with exposure to the intended environmental stressors is the most direct method of establishing the quality of corrosion preventative coatings. For more information click on the technical publications below that were authored by Matco Services Paint and Protective Coatings Group.
Galvanizing Process Recommendations:
Matco Services Inc. recommends the following basic quality controls for your galvanizing operation and galvanized products which includes testing of various lots and documentation procedures be followed and provided to Matco Testing Labs for their review. The diagram below illustrates the hot dipped galvanized steel process.
Source: Google Images, Amazing Grace
Steel Substrate Testing
- Chemical and physical testing of steel to be galvanized should be performed and documented. Chemical analysis should include Si & P
- Surface organic contamination should be determined prior to galvanizing to obtain the best results
Acid Tank Testing
- Total Acid %
- Active Acid%
- pH
- Iron %
- Temperature
Start of shift plus once per lot.
Rinse Tank (water) Testing
- Iron %
- Zinc %
- Chloride %
- Copper %
- pH
- Saturation Index
Start of shift plus once per lot.
Flux Tank Testing
- pH
- Active Flux -Parameters per vendor’s recommendations
- Baume
Start of shift plus once per lot.
Galvanizing Tank Testing
- Chemical Analysis (Zinc, Iron, Lead, Copper, Nickel and Tin.)
- Temperature
Tested - Start of shift plus once per lot
Phosphate Tank Testing
- Bath parameters: Total Acid, Free Acid and Activator Concentration
- Chemical Analysis (Iron, Zinc, Chloride, Sulfur, Phosphate)
- Temperature
- Proposed Clean Process
Tested - Start of shift plus once per lot
The following performance parameters must be determined and documented for each lot:
- Phosphate Coating Weight and Coating Morphology
- Uniformity and Completeness of the Phosphate Coating
- Phosphate Coating Adhesion
Final Product Testing
Samples from each lot should be tested and verified to pass the following requirements; color per cline;s requirements, more than 4 mils of a Galvanize layer. Light-weight phosphate coatings (0.2-1.4 g/m2) to Middle-weight (1.4-7.5 g/m2) or heavy weight depending on color that meets the client’s specification requirements. Pass pre specified hours of exposure to ASTM B117 without exhibiting red rust. The acceptable chloride levels when using the ChlorTest product on the finished product is 0-10 ppm. Analysis of final product cross section for Eta Phase.
Important Notes:
Many phosphate processes are proprietary and it’s absolutely necessary to include their monitoring techniques in the quality assurance program.
The color of finish product depends on phosphate crystal size/phosphate weight, composition of phosphate bath and type of sealant. You need to control these parameters in the bath based on vendor recommendations. For optimum process efficiency, phosphate coating weights should be documented and the results tracked over time.
Control of pH is critical because the phosphate precipitates out from the solution only when the pH at the substrate/liquid interface is in the correct range. Because the pH range is specific to the particular phosphate formulation, vendor recommendations must be followed exactly.
Samples for Testing
Samples shall be parts or scrap or test panels made from the steel, made from the same process or heat and represent the same size and surface area.
Concerns regarding galvanized steel should be directed to Dr. Zee, a NACE certified corrosion engineer specializing in hot-dipped zinc bath chemistries, finished product, galvanized steel failures, protective coatings, corrosion, and the advancement of this material through research and development . He can be reached at the following locations:
Dr. Zee
Office: 412-788-1263 ext. 13
Cell: 412-952-9441
Email:
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
Galvanized Steel Associated Technical Publications Include:
Painted Galvanized Steel Case Histories, by Dr. Mehrooz Zamanzadeh and Dr. George Bayer presented at PACE 2006.
Some Failure Analysis Case Histories in Galvanized Steel Products, M. Zamanzadeh and Ed Larkin internally published by Matco Services, Inc.
Discovering Outgassing Defects, M. Zamanzadeh, internally published by Matco Services, Inc.
A Re-Examination of Failure Analysis, by Dr. Mehrooz Zamanzadeh, Dr. Donald Gibbon, and Edward Larkin, internally published by Matco Services, Inc.
Failure Analysis of Coatings, by Dr. George T. Bayer and Dr. Mehrooz Zamanzadeh, internally published by Matco Services, Inc.
Evolution of Material Analysis, Dr. Gibbon, internally published by Matco Services, Inc.
Please give us a call or send us a service request form and we can discuss your galvanizing issues. We can provide you with a quotation to perform either an on-site or laboratory investigation of your galvanizing and painting problem.
Dr. Zee Dr. George Bayer
Tom Thomas Geoff Rhodes
Ed Larkin Marty Latona
Walter Gretz Sam Scheinman
Dr. Huiping Xu Antonio DiNunno
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Failure Analysis 
