Creators of GAL-5 and GAL-7 Sprinkler Pipe
IDOD Systems, LLC • PO BOX 1635 • Homewood, Illinois 60430
GALVANIZED PIPE AND PROCESS’S
A previous article discussed why galvanized pipe should be used in dry systems; this 2-part article will cover the history and quality issue of galvanized pipe. Before discussing the actual steps and procedures for galvanizing, a few issues should be made clear. In order to be considered galvanized, an alloy layer needs to be formed between the zinc and the steel substrate. An alloy layer is formed when both the zinc and steel are above the melting temperature of the zinc. The zinc quality must be monitored as well as the steel substrate. The steel substrate must be completely clean of dirt, oil, paint, and oxides. Any area not completely spotless, will not be galvanized. There may be small areas coated over a small oxide spot, but the lack of adhesion can eventually break away during fabrication. The thickness of the alloy layer is also an issue – but will be discussed later in this article.
The process for standard hot dipping of steel pipe is over 150 years old. Little has changed in those 150 years. The following flow diagram shows the standard procedure used.
Existing Standard Hot Dip Pipe Process:
Caustic Cleaning – The pipe normally comes to the hot dip facility directly from the pipe mill. Assuming it was rolled specifically to be galvanized, externally and internally it has a black oxide layer, mill lubricant and dirt on it. The caustic cleaning is meant to remove the lubricant and the dirt. However, when putting a 21’-0” foot pipe into a bath of hot caustic cleaner, the assumption made is that the dirt and oil inside the middle of the pipe is being removed. Visual inspection is rarely done. The pipe is then put into a hot water rinse tank to remove the caustic cleaner.
NOTE: Pipe to be hot dipped with OD lacquer is subject to another step of either shot blasting or some kind of chemical paint removal.
Pickling – As mentioned above, the pipe has a black oxide (scale) layer over the surface of the steel. Assuming the dirt and oil has been removed properly, the scale needs to be removed next. Dipping the pipe into hot acid (Hydrochloric or Sulfuric), the scale is slowly removed. The acid removes the scale by converting it from an oxide into hydrogen gas and a salt (Iron chloride or iron sulphate) that is soluble in the liquid solution. That said, when the solubility exceeds 10% to 15%, the salts can and do precipitate out and fall to the bottom of the acid tank – except for inside the pipe. Inside the pipe, the salts fall to the bottom of the pipe where it can shield the oxide below the fallen salts from being removed. According to standards, pipe should be rotated in the pickling tank to assure all areas of the pipe are in contact with the acid. The pipe is removed from the acid at an angle under the assumption that the salts inside the pipe will fall out. In this step, again the condition of the inside of the pipe after the pickling operation comes into question. What assurances are there that the scale internally has been removed successfully? The pipe is then dipped into a hot water rinse tank to remove the acid and any loose scale.
Flux – There are several purposes for the use of flux in a galvanizing operation. If there is a small amount of oxide remaining on the surface of the steel, the flux will help remove it and allow the zinc to bond with the steel. It also acts as a wetting agent on the surface to help the molten zinc easily move over the steel surface – helping to smooth it out. To the readers who have soldered copper fittings, try soldering without the flux. Most of the fluxes used for zinc galvanizing are zinc ammonium chloride based. The problem with a flux system is that the “spent” flux leaves a solid deposit made up primarily of zinc chloride or ammonium chloride. Both are relatively soluble and rinse out easily. However, when combined with zinc oxide deposits that are solids and left in the pipe, these chloride deposits become time bombs. If the galvanized pipe is installed where those deposits can become wet, the zinc chloride is an acid and will start to attack first the zinc then the steel. If a galvanized pipe fails prematurely and there is a high concentration of chlorides, there is a good chance the flux was the cause for failure. The chances for the deposits are very closely related to the galvanizing drainage procedure.
Hot Dip Galvanizing – There are several ways to dip the pipe product into the zinc. After the pipe has been removed from the flux, at best it is about 150o F. In order to form an alloy layer, the molten zinc and the steel surface need to be close to the same temperature (usually about 850o F) – with the zinc in the molten state. Because the pipe is immersed at a relatively cold temperature, it must stay in the molten zinc for some period of time until the steel is brought up to alloy temperature. If removed to soon, there will be no alloy layer, and if left in longer than necessary, the alloy layer will continue to grow in thickness. One of the biggest differences of hot dip processes is the removal of the pipe from the zinc and the drainage of the excess zinc. In the best method, the pipes are pulled up out of the pot at an angle. As the pipe is brought out, it is coming out through an air ring, which is blowing off and smoothing out the outside surface of the pipe. Once the pipe is fully removed, a nozzle is brought to the end of the pipe where super heated steam blows out the inside. The advantage of this system is a smoother surface inside and out, with a minimal chance for the zinc oxide deposits to be left in the pipe. The disadvantages are the potential for removing too much zinc and not being able to meet the ASTM Standards such as A53 and A795. The often-used scenario has the pipes being hung on “fingers” of a rack at a slight angle. The pipes go from one tank of the process to another on these racks. Drainage is strictly gravity. The advantage of this system is only one of cost. The disadvantages are the potential for zinc oxide deposits being left inside the pipe, and the excess zinc left at the bottom of the pipe. The zinc thickness at any given point on the pipe will vary significantly. Another potential disadvantage is the potential for bowing the pipe. When the pipe is being pulled from the zinc bath, the length of the pipe, wall thickness, and the speed at which it is removed are critical. The weight of the molten zinc inside the pipe as it is removed can cause the pipe to bow. The worst case is when the pipe has been pre-fabricated with weldolets and grooves before the process, and the “finger” method is used. Potential dirt and oxide sludge unable to get past the grooved area, heavy zinc in and around the grooved area, barriers for the zinc oxides to be held onto, as well as the potential bowing from the weight make this method the least favorite of the often used methods. There are several other versions of hot dipping, some better than others, but none very good. Remember, this is basically 150 year old technology.
Strip Galvanizing – The modern strip galvanizing process is a continuous process. The atmosphere reducing strip galvanizing process is almost 75 year old technology. The process was invented by Mr. Tadeusz Sendzimir in Poland.
As you can see in the schematic, once the process starts, it continues. Location 5 is an entry looping tower which stores the strip steel and allows the ends of the strip coil to be joined. The main concept of the process is the same as the standard hot dip process, except it has better quality assurances and repeatability. The cleaning process allows for both sides of the strip to be hit with high pressure hot cleaners. The pickling process can be done several ways – from in line acid systems as done in the hot dip method – but again in a strip form where the quality can be controlled, to combinations of shot blasting or atmospheric reducing furnaces. In the end, the strip is completely free of scale to such a degree that flux is not required. In the continuous strip form, the steel can be heated to alloy temperature in a de-oxidizing atmosphere before it comes in contact with the zinc. As a result, when the molten zinc and pre-heated steel come in contact together, there is an instantaneous alloy layer. The strip goes into the zinc under a sinker roll and comes back out through a set of air knives that control the zinc thickness on the strip. The time for the alloy layer to form is seconds compared to minutes and the adhesive properties are thin and flexible - yet tenacious. With the strip feeding through the process at a fixed and controlled speed, the air knives are able to blow off the zinc uniformly. Technology has since added the ability to actually verify the zinc thickness after the air knives and adjust for variances accordingly.
Another key issue in the strip galvanizing process is the aluminum content in the zinc pot. Strip galvanizers can and do use about 0.2% aluminum that helps control the alloy layer as well. The standard hot dip pipe process cannot have the aluminum in the zinc pot because it doesn’t react well with the flux.
The Outside of Pipe Only Galvanizing Process – This technology has been around for almost 45 years. Invented and developed by Mr. Ted Krengel, using the strip galvanizing process concepts, the technology allowed for the outside of a tube to be galvanized in line on the tube mill. The strip could be de-scaled at the steel mill, and all the process needed was for a light coat of oil to be removed, the strip is formed into a tube and welded. Once welded, another quick cleaning and final acid pickling prepares the tube as it goes into an atmospherically controlled induction heating process that brings the tube temperature up to alloy temperature. The heated tube is immersed into a kettle of molten zinc where the alloy layer is also thin and flexible. This process in an “outside of the tube only” process. The inside of the tube is still uncoated, but for industries other that Fire Protection, can be painted internally. Pipe made from this process is not considered galvanized pipe acceptable by UL, FM, or NFPA-13.
The IDOD Process – A continuous in-line process that marries the strip galvanizing process to the pipe making process. This new technology, also invented and developed by Mr. Ted Krengel, has been in production for over 4 years. It has allowed the quality found in the strip galvanizing and OD galvanizing industries to be permitted into the pipe making industry. Starting with Pre-galvanized strip – with its controlled zinc thickness and quality – the pipe is formed and welded as on any other pipe mill. The weld seam is the only portion of the pipe not galvanized. The IDOD Process re-applies the zinc coating both inside and outside over the weld seam. The seam is then re-heated up to alloy temperature with induction heating – in atmosphere. Where the standard hot dip process uses the molten zinc to bring the steel up to alloy temperature, the IDOD Process heats the steel, which re-melts the zinc over the seam – and an alloy layer is formed. The pipe is then quenched quickly. The time at alloy temperature is minimal and the alloy layer is thin and flexible.
The pipe industry does not have any adhesion standard required. The strip galvanizing industry has an ASTM standard – A653 with an adhesion requirement. IDOD has FM approval to claim that it passes the A653 zinc adhesion test.
Quality issues – Standard hot dip pipe generally chips and flakes. The alloy layer is relatively thick and brittle. Strip galvanized products do not chip or flake. The alloy layer is relatively thin and flexible. End products using strip galvanized material rarely – if ever – chip or flake. If the zinc coating on your car or appliances were chipping and flaking where it was fabricated, it would never be accepted. The zinc thickness at any given location on a standard hot dip pipe can vary from much less than acceptable by the ASTM A53 standard (about .003” thick) to zinc so heavy it is thicker than the parent metal where it had been dripping off.
In the photomicrographs shown below, you can see the thicker alloy layer of the standard hot dip process vs. the thinner alloy layer made by the continuous strip galvanizing methods.
Micrograph of Hot Dip Batch Galvanized Standard Hot Dipped Galvanized Pipe
4 oz/ft2 coating: Cross-section @ 250X Grooved – Chipping and Flaking due to thick Alloy Layer.
Photomicrogagh of Continuous Strip Galvanizing Galvanized Pipe produced from the
There are no ASTM Police – According to the NFPA-13 guidelines, pipe claiming to meet ASTM A53 (1.8 Oz/Ft2) or A795 (1.5 Oz/Ft2) is automatically approved for use in Fire Protection sprinkler systems. A study done by IDOD Systems of 14 different galvanized pipe manufacturers – both domestic and foreign – showed that half (7) did not meet the ASTM A53 zinc requirement. There are no ASTM agencies that police whether a company meets the standards ASTM has set. It is an honor system. UL and FM are the policing agencies for the Fire Protection Industry. While UL at this time does not involve itself in zinc quality issues, FM has set the acceptable zinc standards of A53 of 1.8 Oz/Ft2. FM currently has 4 FM approved Pipe Producers (IDOD Systems, Allied Tube and Conduit, Wheatland Tube, and Ispat Sidbec). All 4 must consistently pass the A53 zinc requirements and are subject to quarterly FM audits to the facilities.
As a side note, only one galvanized pipe producer has actually been fully tested by both UL and FM with both the fittings and with zinc on the pipe. Because IDOD’s finished pipe off the mill is galvanized, the pipe was tested with the fittings accordingly. FM has modified their testing and approval standards for future galvanized manufacturers approvals so that a random sample of pipe and fittings must be tested before approval after the pipe has been galvanizing.
Galvanized pipe has been around for the 150 years mentioned. When deciding on the use of a galvanized pipe, consider the process used and the quality available. There are choices available.