Thermal Spray Coatings of Zinc and Aluminum Zinc Metallizing

Thermal Spray Coatings of Zinc and AluminumZinc Metallizing

TS Coating Galvanizing / 2

Prepared byJim Weber - Sulzer Metco (US) Inc. – Westbury, NY

Thermal Spray Materials Development

Systems Engineering

Equipment Research & Development

Field Service Manager

Training Manager

Quality Assurance Manager

Currently the Product Line Manager for industrial markets, focusing on combustion (wires, powder, & HVOF) and arc equipment

Latest industry focus - working with NACE International (National Association of Corrosion Engineers) and the petrochemical industry to solve Corrosion Under Insulation (CUI) problems with Thermal Spray Aluminum (TSA) coatings and with the Gas Technology Institute (GTI) solving natural gas transmission and metering station corrosion problems.


Information A brief introduction to thermal sprayed coatings are of Zn (TSZ), Al

(TSA), and their alloys (85/15 Zn/Al), and how they provide long term corrosion protection

Some examples of where are these coatings are used and how they have performed

A brief introduction to galvanizing and its comparison to thermal spray coatings

An overview of the most popular types of equipment that apply these coatings

Question & answer session


What is Thermal Spray? Thermal spray is NOT a welding process

Thermal spray coatings are a melted, or softened ceramic, metallic, or polymer materials are transported by a gas stream to a properly prepared substrate

Heat Source AcceleratedDroplet

Coating

Substrate

Or

Material Feedstock


What is Thermal Spray? These coatings are typically mechanically bonded to a grit blasted

surface

Metals that feed into the thermal spray gun are the actual coatings. There are no solvents or VOC's.

The coatings are similar to the metals being sprayed, however there are some important differences

1. There are metal oxide stringers and porosity in the coatings

2. Metal particles in the coating create a layered effect within the coating structure

3. Due to the rapid cooling of the metal particles as they adhere to the substrates, thermal sprayed coatings have unique crystalline structures not normally found in wrought metals

Almost any material can be thermal sprayed onto almost any substrate


What is Thermal Spray? Thermal spray, especially with soft metals sprayed with the combustion

wire equipment, is a relatively cold process. Substrate temperatures seldom reach >200F/95C.


How thermally sprayed coatings of Zn and Al, combat corrosion

For atmospheric, buried, and marine environment corrosion protection, Zn (TSZ), Al (TSA), and their alloys have proven that they provide long term corrosion protection and outperform most all other methods.

Anodic (TSZ/TSA) metal coatings applied to steel cathodes (more noble than Zn or Al), are referred to as cathodic or sacrificial protection coating systems.

These thermal spray coatings provide corrosion protection by excluding the environment (or electrolyte) and acting as a barrier coating (like paints, polymers, and epoxies), but unlike typical barrier coatings they also provide sacrificial anodic protection.


Typical thermal spray applications or Zn, Al, and their alloys Hundreds of bridges have been thermal sprayed (metallized) over the past 100 years. There are

many document cases of >50 years of corrosion protection provided by thermal spray coatings.

Zinc and zinc alloys are also sprayed directly onto concrete to protect the steel rebar within.

http://upload.wikimedia.org/wikipedia/en/9/9d/ForthRoadBridgeFromNorth_TakenByEuchiasmus.JPG


Typical thermal spray applications for Zn, Al, and their alloys


Typical thermal spray applications for Zn, Al, and their alloys

Carbon steel equipment (petrochemical facilities, power generation, Naval ships, etc.) are coated with TSA, TSZ, and 85/15.

Petrochemical users expect TSA coatings to provide >20 years with no maintenance or corrosion inspections.

A TSA coating has been operating in severe petrochemical vessel operating conditions. One coating in particular has protecting carbon steel equipment operating in cyclic service of ambient to 350F/175C, in contact with wet insulation, for >50 years with no visible corrosion .

TSA coatings are also specified as a solution to stress corrosion cracking (SCC) of austenitic stainless steel.

Offshore oil rigs operating in severe conditions rely on thermal spray aluminum for corrosion protection. >20 service life with no maintenance has been documented.

These coating systems meet specifications from organizations such as NACE, SSPC, AWS, US Navy, US Army Corps of Engineers, and many others.


Petrochemical applications






Navy corrosion applications The Navy has many documented uses for thermal spray coatings of all

types, including corrosion coatings

High temperature (>900F/480C) steam valves and associated piping have been protected by thermal sprayed aluminum on Naval ships. It is documented that these coatings lasted longer than 5 years in areas where paint coatings had lasted only months.


Hot-Dip Galvanizing Hot-dip galvanizing is essentially a brazing process. Steel is cleaned

(mechanically and chemically) and then heated to >850°F in a molten zinc bath. The molten zinc reacts with the surface of the steel and forms a coating of various layers of zinc and iron alloys, depositing a layer of material that is approximately 40% pure zinc.

A. Degreasing in a hot, alkaline solution B. Rinsing thoroughly in a water rinse C. Pickling in a hot, acid bath D. Rinsing thoroughly in a water rinse E. Prefluxing in a zinc ammonium chloride solution F. Immersing the article in the molten zinc through a molten flux cover

(usually zinc ammonium chloride) G. Finishing (dusting with ammonium chloride to produce a smooth

finish). H. Sometimes step F is followed by immersion of the zinc coated article

in a quench bath which may contain a dichromate solution.


Hot-Dip Galvanizing


Hot-Dip Galvanizing

Galvanized Coating

Thermal Spray Coating


Galvanizing VS. Thermal Spray Pure aluminum (>99%), zinc (>99.9%), and zinc-aluminum alloys may

be sprayed, whereas galvanizing is never pure zinc, it is Zn/Fe matrix. Also, the molten zinc bath is often contaminated with pickup from materials that are dipped into it.

Aluminum and the 85/15 zinc-aluminum alloy coatings are not available by hot dip galvanizing.

Aluminum and Zn/Al coatings protect steel better than pure zinc in marine and industrial environments.

When metalizing, the surface being coated is not preheated and it remains at a low temperature, with local temperature never exceeding about 250-300° F. Because metalizing is a "cold process" when compared to galvanizing’s >850°F temperature, there is virtually no risk of weld damage or distortion of the steel due to high temperatures or overheating


Galvanizing VS. Thermal Spray The acids, fumes, contaminated rinse water, and other byproducts of the

galvanizing process are considered hazardous. Thermal spray waste is generally metal dust, which when applied in a shop environment, is collected by specially designed dust collectors that remove up to 99.99% of the generated dust, down to .12 microns, and may be recycled. Metal dust generated while spraying onsite may be ignored, swept up, or collected with grit blasting containment and collection systems depending on local requirements and conditions.

Due to environmental concerns, new galvanizing lines are no longer permitted in many states, including NY state. The lines that are currently running must be updated to current environmental regulations within a set time frame, as dictated by local, state, and federal codes for hazardous waste and emissions.

Most paint and powder coating applicators can easily upgrade to thermal spray coatings. It is difficult, or impossible to upgrade to a galvanizing line, therefore fabricated parts must be shipped to approved galvanizers and then shipped back to approved painters/powder coaters.


Galvanizing VS. Thermal Spray Because galvanized coatings must be dipped in a tank, parts are limited

to the size of the molten zinc galvanizing tank.

While galvanized coatings are difficult to top coat and may require special surface preparations (acid etching, phosphating, etc.) to allow topcoats to adhere to them, thermal spray coatings 5-15% porosity make them a perfect surface for paints and powder coats. The coating soaks up topcoats like a sponge, creating a tremendous bond between the metal coating and the top coat.

Thermal spray coatings are also less susceptible to out-gassing, a phenomenon that causes bubbling and loss of adhesion of topcoats.

Field repairs to galvanizing can only be done with paint, which does not last nearly as long as the galvanized coating. Typical thermal spray equipment is portable and easily used onsite in the field. An added benefit to this equipment is it’s low cost, less than $12,000 for a complete portable system.


Galvanizing VS. Thermal Spray “The Use of Thermal Sprayed Zinc Alternative to Hot Dipped

Galvanizing” Joseph G. Radzik, Director of Engineering Research & Development, Tyco Fire & Building Products

Representative samples of Thermal Sprayed Zinc and Hot Dipped Galvanized ductile iron castings conforming to ASTM2 A 536, Grade 65-45-12, were subjected to a 5% salt spray fog test at 95 °F in accordance with ASTM B 117. Coating thickness was measured in the range of 3.4 to 5.5 mils for all samples evaluated, which conforms to the industry requirement for grooved piping products. An independent laboratory NADCAP accredited in materials and nondestructive testing methods performed the testing. After 120 hours exposure, the Hot Dipped Galvanized samples exhibited red corrosion. The Thermal Sprayed Zinc samples did not exhibit red rust corrosion. Testing was continued on the Thermal Sprayed Zinc samples for an additional 72 hours for a total of 192 hours and examined for signs of red rust. None of the Thermal Sprayed Zinc samples exhibited this condition and testing was stopped.


“Ski-Lift Maintenance: Wire Arc Spray vs.Galvanizing,” M. Bhursari and R. Mitchener, SSPC International Conference, 1998.

This paper reviews the use of wire arc spray zinc vs. galvanizing on ski lifts. The authors discuss a case study in which painted lifts required repainting every 3 years, hot dipped lifts showed signs of corrosion in fewer than 5 years and thermal sprayed ski lifts exhibited no corrosion after 5 years. It was estimated that the wire arc-spray zinc coating, depending upon the thickness, would have a life expectancy of 20 years with minimal maintenance. The authors concluded that thermal spray coatings were more resistant to abrasion and wear than thin galvanized coatings.


Combustion Wire System














Twin Wire Arc System















Thermal Spray Coatings of Zinc and Aluminum Zinc Metallizing

Documents