Zinc Wire for Thermal Spray Metalizing

Thermal Spray Metalizing with the 85/15 Alloy

The Benefits of Thermal Spray Metalizing

The 85/15 Alloy

In 1986, Platt Brothers began producing an alloy of 85% zinc with 15% aluminum wire for the thermal spray process. It is a difficult alloy to make, but, after years of study and success in Europe, Platt believed that this alloy would benefit the corrosion industry, especially in the fight against sodium chlorides on our nation's highway bridge structures.

Why bother with this alloy? The following excerpts from research done worldwide will attempt to explain the benefits of 85/15, why the alloy was developed, the drawbacks of the alloy and information from equipment manufacturers about the ability to spray 85/15.

The Development

85/15 is an alloyed wire of 85% zinc and 15% aluminum by weight. This combination and percentage by weight was derived from development studies of the behavior of both zinc and aluminum. In 1966 the French Electricity Research Department published a paper describing positive behavior of a zinc alloy with 15% aluminum placed for 33 months inside forced conduits. Further studies of different combinations of zinc and aluminum revealed that the corrosion potential of zinc is more negative than that of iron. The anodic reaction gives sacrificial protection to steel. Aluminum behaves as an inert coating, thereby creating a passive type protection to steel.After various combinations, the results found that an increase in the aluminum weight content of the alloy lead to an increase of aluminum volume. That is, 28% by weight corresponded to 50% by volume. At this high content or above, aluminum became a large volume of the coating. When combined with zinc, the zinc was not able to provide the coating with a cathodic protection effect, but was still sufficient enough to avoid the problem of underrusting for a short period of time. However, the zinc's sacrificial protection was not strong enough to hinder the corrosion of the steel in areas of defects, thus behaving as a pure aluminum coating. Simply put, any coating using more than 28% by weight of aluminum acts as a pure aluminum coating.

Examination of cross sections of the 85/15 coating lead to the discovery of a two phase, evenly distributed coating structure consisting of large elongated particles containing mainly aluminum surrounded by tiny particles of zinc. From a structural standpoint, the coating behaves as a zinc coating which would be reinforced by a weaving of the more inert aluminum. Also, after electrochemical analysis, it was concluded that the two phases combine the fundamental electrochemical properties of each of the two components.

In 1965, CEFRACOR, the French Anti-Corrosion Centre carried out a 10 year test on 85/15 in conjunction with different types of paint coatings in both atmospheric and immersion testing. Their conclusions have been documented to these points:

  • 85/15 has very fine pores and therefore, rapid natural sealing.
  • High electrochemical potential and hence constant sacrificial capability comparable to that of zinc.
  • An aluminum base which slows down zinc dissolution and increases protection duration.
  • From both technical and aesthetic standpoints, 85/15 can be left unsealed, used in smaller thicknesses than those required for zinc and very much smaller than those required for aluminum.
  • Subsequent tests have shown that 85/15 has excellent corrosion performance in chloride or sulfur dioxide environments both atmospheric and in underground corrosion applications.

The Benefits

Aside from the benefits of having a dual phase coating 85/15 sprayed by both flame and electric arc metalizing equipment produces a denser coating than pure zinc or aluminum. The maximum service temperature of the alloy (600°F, 31 5°C) is also much greater than zinc (120°F) increasing its usability in applications such as steel storage tanks.

Because of the difference in density of the metals the square foot coating coverage per pound using 85/15 compared with pure zinc is increased by over 22%. Given a 6 mil coating thickness, Zinc covers 3.5 sq. ft. per pound whereas 85/15 covers 4.3 sq. ft. per pound.

Caution:

There are applications where 85/15 either does not work well or extra care should be taken when using the alloy.

Those areas are:

Direct contact of an 85/15 coating with freshly poured concrete. should be avoided until the concrete is set. Because of the coating structure (see above), the aluminum element of the coating is very reactive with extreme alkaline conditions like those encountered in fresh poured concrete. Hydrogen gas will form and degrade the concrete surrounding the coated steel.

Testing of the 85/15 coating on concrete to protect steel reinforcing bars indicates that 85/15 does not work as pure zinc performs on concrete. The 85/15 coating bubbles and disbond in the more anodic areas. Platt does not recommend 85115 for this application. Continued testing looks promising using alloys with small amounts of aluminum and studies are ongoing to determine the cause of the 85/15 disbonding.

85/15 and the FHWA

Much has been said as to the effectiveness of 85115 over pure zinc in the ongoing FHWA studies. Here are the facts from the FHWA Report, Environmentally Acceptable Materials for the Corrosion Protection of Steel Bridges, Publication No FHWA-RD96-058. Metallizing systems (Zinc, Aluminum and 85/15) provided consistently the best corrosion protection performance. All metallized coatings tested showed no corrosion failure in the aggressive salt rich environments over the 5 to 6.5 year exposure periods.

In the 5 year tests, metallized systems have provided excellent corrosion protection at all three locations. There was exceptional overall appearance of the 85115 panel. 85115 was the only coating extended with a 30 and 60 year life for life cycle cost analysis.

Much of the report discusses metallized coatings in general. However, the report does state:

"That although the zinc provides good corrosion control, the appearance of its own corrosion products may be aesthetically unacceptable. Exposed zinc metallizing produces highly visible white zinc corrosion products. Alloying the zinc with small amounts of aluminum maintains the sacrif cial properties of the zinc while reducing the amount of voluminous white zinc corrosion products. Alloying also increases the hardness of the coating. Aluminum metallizing is typically employed in high-temperature areas where corrosion control afforded by coatings would be jeopardized by low melting temperature. All three of the alloy systems can he either sealed or unsealed."

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