NON LIQUID COATINGS
- 29 May 2017
- Posted by: Stm Coatech
- Category: Educational Articles
NON LIQUID COATINGS
- HOT DIP GALVANIZING
a.Introduction:Hot-dip galvanizing coats iron or steel with a thin zinc layer by passing the steel through a molten bath of zinc at a temperature of around 438-460ºC. The process results in a metallurgical bond between zinc and steel with a series of distinct iron-zinc alloys.
Hot-dip galvanized coatings usually have three distinct layers,each consisting of different amounts of zinc and iron. The ETA or outer surface which contains %100 zinc is not considered a separate layer. Below are the various layers and the zinc/iron content breakdown:
=ZETA (%94 Zn %6 Fe)
=Delta (%90 Zn %10 fe)
=Gamma (%75 Zn %25)
b.Process: There are several major stages in the hotdipping process. The three main steps are:surface preparation, galvanizing and posttreatment,each of which is discussed in these sections.
The first step, surface preparation is to obtain the cleanest possible steel surface by removing all of the oxides and other contaminants. This is achieved in various ways based on the project specification.
Solvent cleaning and abrasive blasting are frequently specified.This is good for hot-dip galvanizing because it not only covers the cleanliness desired but also creates a measurable anchor profile. Therefore, either abrasive blasting or caustic cleaining are likely to be the first step based on the specification and the plant equipment set-up:
=Caustic cleaning immerses steel in a caustic solution to remove the dirt,oil and grease from its surface. Rinse the steel with water after degreasing.
=Pickling immerses the item in an acid tank, filled with either hydrochloric or sulfuric acid, to remove oxides and mill scale. Once these are removed, it is rinsed again with water.
=Fluxing cleans the steel of any oxidation developed after pickling,which creates a protective coating to prevent oxidization before the steel reaches the galvanizing kettle. Generally, the fluxing process is one of two types:
-Combination of zinc chloride and ammonium chloride. It is contained in a seperate tank and is slightly acidic.
-Top flux floats on top of the liquid zinc in the galvanizing kettle but serves the same purpose.
-Zinc Bath(Hot-Dip Medium)
The galvanizing kettle contains zinc specified to ASM B 6 (or a similar standard),that specifies one of three different grades of zinc that are each at least %98 pure. Sometimes other metals are added to the zinc melt to promote certain desirable properties in the galvanized coating. The inspector must not make recommendations on bath composition.
Galvanizing kettles typically operate at temperatures ranging from 438-460ºC, at which point the zinc is in a liquid state. The steel products are immersed into the galvanizing kettle and remain until the steel’s temperature reaches the temperature required to form a hot-dip galvanized coating.
Once the steel is removed from the galvanizing kettle, it may receive a post-treatment to enhance the coating. Post treatments are done to produce one or more of the following:
=Reduce coating thickness. This is done by reducing the amount of molten metal that adheres as the article leaves the bath. Roll,wipe,centrifuge-or air-blast the steel to accomplish this.Do this while the coating is still molten. Chromate,phosphate,or light-roll and roller-level the steel to improve the properties or appearance of the coating.
=Change the properties of the coating. Annealing hot-dipped zinc coatings converts the whole of the coating into an alloy.
c.Inspection: Inspection and test methods for hot-dip galvanizing are specified in standards such as:
=ASTM A 123/A 123M
=ASTM A 153/A 153 M
=ASTM A 767/A 767M
This chapter focuses on the simpler,more common inspection issues.
The basic finish requirements for galvanized coating include:
=Free of gross imperfections:
-Flux,ash or dross inclusions
The term “smoothness” is relative, so it is the job specification that sets the tolerances for smoothness.
Differences in the luster and color of galvanized coatings do not significantly affect corrosion resistance and the presence of spangle or “zinc crystals” has no effect on the coatings performance.
Some of the common problems seen during visual inspections are discussed below.
Article in Contact: The zinc in the galvanizing bath needs free access to all surfaces.Keep articles entering and passing through the galvanizing bath out of tight contact with each other.
Rough Coatings: Rough,heavy coatings means galvanized components with markedly rough surfaces. This includes coatings with just a rough surfacce or those with a groove-type surface. A rough coating usually is caused by excessive growth or unevenness of the alloy layer.
Excess Aluminum: This condition (sometimes called black spots) may occur if the aluminum content of a bath,over which a flux blanket is used,is too high. Avoid by keeping aluminum content below %0.01.
Dross Protrusions: Dross protrusions and stipple are small, hard lumps on an otherwise normal galvanized surface. The protrusions result from agitation of the dross layer at the bottom of the bath or from dragging the item through the dross layer.
Lumpiness and Runs: A lumpy and uneven coating results when the item is withdrawn too quickly or the bath temperature is too low to allow surplus zinc to run back into the bath.
Uneven Drainage: Drips are removed by filing or other means, if required. Look for voids where drips have been carelessly removed or knocked off.
Flux Inclusions: Flux inclusions originate in several ways. Stale or spent kettle flux tends to adhere to the steel instead of separating cleanly when dipped. Either way, the inclusions are associated with bare spots in the coating.Black spots formed by flux inclusions are distinguishable from dirt smuts, splash marks and other less harmful types of contamination by their tendency to pick up moisture.
Ash Inclusions: Ash inclusions are zinc ash,an oxide film that sometimes develops on the surface of the galvazing bath. As with flux,ash may burn onto the steel during dipping, or be picked up from the top of the bath during withdrawal.
Dull-Gray Galvanized Coating: This gray or mottled apperance develops during cooling and is caused by diffusion of the zinc-iron alloy phase to the surface of the coating.
Rust Stains: Rust stains can be caused by seepage fromjoints and seams after galvanizing or by storing galvanized materials under or in contact with rusty steel.
d. Repairs: All project specification should provideguidlines on repairs. Some specifications also reference a standard such as ASTM A-780 Standard Practice for Repair of Damaged Hot Dip Galvanized Coatings.
e. Storage: There are a few issues with storing hot-dipped galvanized items.Inspectors need to be aware of them and understand the issues are not generally cause for rejection:
=Rust Stains: As mentioned in an earlier section,these are caused by seepage from joints and seams after galvanizing or when galvanized materials are stored under or in contact with rusty steel.
=Wet Storage Stains: Wet storage stains are a buildup of zinc oxide and zinc hydroxides on a galvanized surface. A medium to heavy buildup of white corrosion products must be removed; otherwise the essential protective film of basic zinc carbonates cannot form in affected area.Remove light deposits by brushing with a stiff bristle brush.
When this occurs a significant amount of coating has been lost to corrosion and the service life is decreased.
- Spray Metalizing / Thermal Spraying
a.Introduction: Spray metalizing is the general term used for the process of coating metal onto the surface of non-metallic objects. However, this term has grown to include applying a coating by spraying molten metal on steel substrates as a form of corrosion protection.
In spray metalizing, as in coating,surface preparation is critical.Surface preparation for thermal spraying generally requires a minimum NACE 2/SSPC-SP10 or ISO SA 2.5 with an angular profile.
This section presents the four application processes inspectors need to know.
=Flame Spraying: Flame spraying is part of the wider group of coating processes known as thermal spraying systems. To flame-spray,feed oxygen and a fuel gas,such as acetylene,propane or propylene,into a torch and ignite to create a flame.
=Arc Spraying: The arc spray process insert two wires into the torch so they contact each other at the nozzle.Placing an electrical load on the wires causes the tips of the wires to melt when they touch. A carrier gas such as compressed air or nitrogen strips the molten material off the wires and transports it to the surface.
=Plasma Spraying: The plasma spray process is considered the most versatile of all the thermal spray pprocesses. During operation,gases such as argon,nitrogen,helium or hydrogen pass through a nozzle.
=High-Velocity Oxyfuel Spraying: The high-velocity oxyfuel (HVOF) process was introduced only 20 years ago.This process expands application possibilities for thermal spraying into areas previously not possible. HVOF spraying injects a combination of process gases into the spray gun’s combustion chamber at high pressure where it ignites.The gas velocities achieve supersonic speeds,so the melted powder also accelerates to supersonic speeds.The results are the densest thermal spray coatings available.
In service,corrosion products of zinc or aluminum develop as the porous coatings begin to corrode.In time, porous coating essentially seals itself with its own corrosion products.To remedy this, the specification often calls for application of a sealer or sealer plus top coat after the application.Sealers for thermal spray coatings (TSC) are low-viscosity,clear or pigmented paints formulated to flow over and absorb into the natural pores of the TSC.Succesful sealers include thin coats of vinyl,PVBA etch primers (generally followed by at least one more coat) and aluminum-pigmented silicone sealers.
e.Spray Metalizing Inspection
Coatings inspectors on thermal spray projects perform inspections and documentation just like conventional coatings. Major concern are:
-Surface preparation (cleanliness and profile)
-Moisture-free substrate verification
-Application procedures and techniques
-Applicationof sealers and topcoats when specified
-DFT measurements (DFT gauges)
-Handling and storage of finished products
Sherardizing is a method of galvanizing also referred to as vapor galvanizing.This process applies a layer of zinc to the metal substrate by heating the object in an airtight container with zinc powder to approximately 300-400ºC the temperature at which the zinc diffusion bonds with the subsrate.
Sherardizing is ideal for small parts and parts that have inner surfaces to coat.In this process,the temperature that the parts reach does not exceed the meltingpoint of zinc,which makes this a dry process.
Aluminizing or aluminum diffusion alloying is a high temperature chemical process in which aluminum vapors diffuse into the surface of the substrate and forms a new metallurgical aluminide alloy.It is an economical way to inhibit corrosion of steels,stainless steels and nickel alloys that operate in severe high temperature enviroments.The aluminide alloys formed during the process contain a minimum of %20 aluminum.
Another method to aluminize is hot-dip aluminizing. The compositions vary among manufacturers and much of the work is proprietary.It is similar to the galvanizing process except the hot-dip bath contains aluminum.
- Inspection of Powder Coatings
Powder coatings films are created when finely pulverized plastics are applied to prepared steel surfaces and fused together by heat. The powders may be thermoplastics or thermosets. Thermoplastics melt, flow and form a film with no change in chemical composition. They are often applied as thick films. Examples include polyethylene, polypropylene, nylon, polyvinly chloride and thermoplastic polyesters. Conversely, thermosets melt, flow and cross-link during curing. They can be applied as thinner films. Examples include epoxy, arcylic, polyurethane and hybrids. Perhaps the most common thermoset powder coating is the fusion-bonded epoxy or FBE. Powder coatings are applied using fluidized beds, electrostatic spray, hot flocking and thermal spray and are used to protect appliances, outdoor furniture, farm implements,lamb housings, rebar, piping and building products.
Inspection of powder coating application includes the powder material itself, surface preparation,curing, appearance, thickness and discontinuty. For thick film powder coatings applied to pipe and other substrates that may be subject to impact damage, a hardness test may be required.
The inspector typically verifies proper storage of the powder material, just as he world liquid coatings, including clean and dry areas that are maintained within the manufacturer’s specified ranges for temperature and humidity. The manufacturer and type of powder and the batch or lot no, should be recorded. The currency of the shelf life of the batch should also be verified prior to use.
The coatings inspector will also verify the degree of surface cleanliness and the depth of surface profile meets the requirements of the project specification. Conversiton coatings such as iron phosphate,zinc phosphate and certain chromates can be used to enhance the adhesion of powder coatings and to provide corrosion protection. The coatings inspector may be required to monitor the application of the conversion coatings prior to application of the powder coating.
The coatings inspector may verify conformance to the powder manufacturer’s curing prosedures, including oven dwell time and temperature, the substrate type and the temperature of the coated part. Specially formulated thermosetting powder coatings may use ultraviyolet(UV) light,rather than heat to cure the film.
Finally, the powder coated item can be visually inspected for overall appearance, coating thickness; discontinuities, holidays or pinholes and Durometer hardness on powder coated pipe and other substrates subject to impact damage during installation. Other methods for assessing cure may also be invoked by the powder coating manufacturer or the contract documents.