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Up Topic Welding Industry / Metallurgy / Stainless Pipe Weld Sugar/Oxidation
- - By DannyAgudelo (*) Date 06-03-2018 14:33

i have a question thats been bugging me fo a while and cant seem to find an exact answer.
Technically what happens that makes a stainless weld get the defect known as sugar when not back purged. Ive read a couple of online posts regarding these and all of them mention the oxides melting points but im not sure if i have the correct grasp on the matter, so would appreciate if someone can give me a more detailed explanation of why these happens.

Thank you
Parent - - By 803056 (*****) Date 06-03-2018 23:10
Sugaring is the term used when one is at loss of the correct terminology. It is simply oxidation on the root side of the welded joint because the root side wasn't protected from oxygen. The blackened surface is simply oxides of chrome, iron, and nickel (assuming it is austenitic stainless). It is a surface issue that has little no impact on the mechanical properties, but it can be an issue if the surfaces are exposed to an electrolyte. Galvanic corrosion results because the surface that is oxidized is "different" from the surrounding base metal and has a different electromotive potential.

Contrary to an article that appeared in the Welding Journal several months ago, the oxide is not evidence the stainless steel is now carbon steel because all the chrome has been burned out. . That is a misconception. It is something I would expect to hear from the guy watching FOX News in the White House, but not from a knowledgeable person.

Parent - - By DannyAgudelo (*) Date 06-03-2018 23:51
But exactly what is it that causes this in stainless while not present in for example carbon steel?
I mean in a more technical way, more detailed.
Parent - - By 803056 (*****) Date 06-04-2018 07:17
All metals will oxidize when exposed to oxygen. The rate of oxidation is like any chemical reaction, it progresses more rapidly as temperature increases.

Some metals limit the amount of oxidation by forming layer of oxide product that protects the underlying metal. In the case of stainless steels the formation of the oxide layer is called passivity. If scratched, the oxide layer immediately reforms to protect the underlying metal. The chrome content of the alloy must be on the order of 11% or higher for the protective oxide layer to form.

Aluminum is another metal that responds to exposure to oxygen in a similar manner. In the case of aluminum it is said that 90% of the maximum oxide layer forms in the first 24-hours. That's why one must clean aluminum alloys just before welding. The window of opportunity is relatively short if optimum weld quality is the goal.

The oxide layer forms very quickly with stainless steels as well, but the oxide layer melts at a temperature that is much closer to that of the base metal. In contrast, the aluminum melts at a temperature much higher than the base metal. That being said, even though the stainless steel looks "clean", the oxide layer is present. The toes of the stainless weld will be somewhat rounded if the joint isn't wire brushed immediately before welding. If the joint is wire brushed immediately before welding, the toes of the weld bead will wet in much better and blend in with the base metal with a smoother transition.

The rate of oxidation increases with temperature. When the metal is molten, as in the case of welding, oxidation is immediate if the weld pool isn't protected. The means of protecting the molten weld pool can be by using a physical barrier, i.e., a slag system, or a gas. Depending on the metal the gas can be an inert gas or an active gas or a mixture of active and inert gases. The active gas doe allow a limited amount of oxidation to occur in the case of carbon steels. The limited oxidation changes the surface tension characteristics and can affect the shape of the weld reinforcement and reduce the tendency to produce undercut along the weld toe.

In the case of stainless steel, carbon steel, aluminum, and other base metals, the root of the weld can easily oxidize if it isn't protected. In the case of shielded metal arc welding, the flux covering reacts with the molten metal forming a layer of slag that provides some protection against oxidation. In the case of gas tungsten arc welding, the inert gas may not be sufficient to protect the root surface from oxidation if the only source of shielding gas is from the torch located on the face side of the joint. The gas from the torch must be supplemented by a backing gas. There are several ways to shield the root surface, one of which is to flood the piping system with shielding gas to displace oxygen from the system. The "purge" must be sufficient to displace the oxygen to reduce it to a safe level. The safe level is dependent on the base metal being welded. In the case of aluminum, the oxide layer protects the underlying molten metal because its melting temperature (of the oxide) is so much higher than that of the base metal. A lot of aluminum is welded without the use of backing gas when the rough oxide layer isn't an issue. On the other hand in the case of other metals, the oxide layer presents a problem. In the case of austenitic stainless steel, the oxide layer can reduce the corrosion resistance if the piping is used to convey liquids that are also electrolytes. Galvanic corrosion can corrode the system very quickly. If the corroded layer can be removed either mechanically or chemically, the corrosion resistance can be improved and equal to that of the unaffected base metal.

Corrosion is a chemical process. Some metals form a protective layer of oxide that inhibits further corrosion. Some metals don't, such as in the case of carbon steels. The oxide layer of plain carbon steel offers little in the way of preventing further corrosion. If the carbon steel is alloyed with sufficient chromium, the oxide layer forms a protective layer that inhibits further corrosion. However, if the oxide layer is not uniform, the corrosion potential increases because there are "chemical" differences from one area to the next. If the system is dry, corrosion isn't usually a major issue, but if the system is wet and liquid is an electrolyte, corrosion can take place rather quickly.

The oxide layer that forms limits further oxidation. Depending on the level of shielding, the extent of oxidation is somewhat limited to relatively heavily oxidized. Improving the level of shielding will reduce the oxidation observed. The level of oxidation permitted is dependent of the nature of the service and the level of corrosion resistance needed.

If you are interested in more detailed explanation, you would have to study how metal oxidize and corrode.

Parent - By jwright650 (*****) Date 06-04-2018 11:07 Edited 06-05-2018 14:53
"All metals will oxidize when exposed to oxygen. The rate of oxidation is like any chemical reaction, it progresses more rapidly as temperature increases."-quote

"Rust" short

Galvanic corrosion is a tough subject to understand.
Materials are listed on a galvanic chart ....

Everything is subject to the flow of electricity (at the atomic level) and some of the materials will corrode faster depending on where they fall on the galvanic chart while present with other materials.
ie. Zinc anode rod in a hot water tank
The zinc rod is the sacrificial element. Note on the chart that zinc is less noble than steel (the hot water tank). The zinc anode rod will "rust" away first preventing the steel from rusting as quickly, but as soon as the zinc is depleted, the steel will start to rust rapidly.

edit: whoops, I got off track from what the OP was asking about.....I should read closer
Parent - - By DannyAgudelo (*) Date 06-05-2018 00:28
So basically sugar is a thick oxide layer that occurs because the high temp increases the rate of oxidation...
How come the oxidation that occurs if the weld is unprotected decreases the corrosion resistance, while the regular oxide layer prevents it?

Thank you for your time and exellent answer by the way.
Parent - - By 803056 (*****) Date 06-05-2018 00:42
The oxide that forms at room temperature is basically chromium oxide, whereas the oxidized root include chromium, nickel, and iron oxides. That being the case, there is a chemical difference between the typical surface oxide and the oxidized root.

Parent - - By DannyAgudelo (*) Date 06-05-2018 01:40
Why is it that at room temperature the oxide that forms is chromium oxide, instead of iron oxide for example?

And is it correct that the defect know vulgar as sugar, is a thick layer of oxide from the various elements in the alloy?
Parent - By 803056 (*****) Date 06-06-2018 00:56
If there isn't sufficient chrome in the alloy, you will get iron oxide and you will not get the corrosion resistance provided by "stainless steel."

Parent - By Jamie Hunter (*) Date 06-05-2018 12:12
Yeah, Al's correct. Sugaring is just oxidization. Most codes don't even make mention of it as being problematic or viewed as a weld defect... though you should really backpurge anyway. It's not loss of chromium, that would be more like carbide precipitation.
Up Topic Welding Industry / Metallurgy / Stainless Pipe Weld Sugar/Oxidation

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