#CHROMIUM OXIDE LAYER HOW TO#
#CHROMIUM OXIDE LAYER MOVIE#
#CHROMIUM OXIDE LAYER FREE#

#CHROMIUM OXIDE LAYER FREE#

These processes use nitric or nitric and hydrofluoric acid solutions, both of which contain oxygen, to remove free iron and ensure that all chromium at the surface bonds with oxygen, maximizing corrosion resistance. Some applications also call for postweld pickling or passivation (please note, pickling a part also passivates it). The lesson here is that the time you spend on postweld cleaning is time well spent. Any slag, spatter, or silica islands provide a potential site vulnerable to corrosion initiation. This explains why pharmaceutical parts require mirror finishes. Next, consider that coarse surfaces are more susceptible to corrosion and contamination than smooth surfaces. Many fabricators have howled in agony after realizing they impregnated their stainless weldment with iron unintentionally. When you are performing mechanical cleaning with wire brushes, grinding discs, chipping hammers, or blasting media, be sure these tools are used specifically for and dedicated to stainless steel to prevent cross-contamination. The type of cleaning depends on the final application and relevant codes and standards. Otherwise, the weld area is subject to rusting. Chromium oxide forms immediately when the chromium atoms in stainless steel are exposed to oxygen in the atmosphere.Īs a general rule, all stainless weldments require postweld cleaning to ensure that a surface’s chrome-oxide film is intact. However, the root cause of this is excess oxidation.Ī microscopic layer of chromium oxide provides stainless steel with its “stainless” characteristic. Fabricators use the slang term “burning the chrome out,” because the darker colors often indicate excess heat input. Layers that have a silver to straw color are good, while heavier layers that progress from a purple/bluish tint to a gray/black tint are not so good. Heat tints occur because the oxygen in an active gas or the atmosphere forms a heavy oxide layer, consuming the chromium that is immediately below the top surface layer. While metal artists and hobbyists might be attracted by the heat tint generated during welding, those colors send signals to the fabricator about how much oxidation took place during the weld. The weld doesn’t need to be strong, but it does need to be watertight. As a result, manufacturers of heat exchangers put a seal weld, usually with gas tungsten arc welding (GTAW), around each tube. Corrosion initiates if fluids seep into the gap between the tube and tubesheet because oxygen has no access to that area to re-form chromium oxide. The sheet has hundreds of machined holes to support bundles of tubes, and cooling water or steam flows around the tubes. As a result, once corrosion initiates, it progresses rapidly.Ī good example of this is the pitting or crevice corrosion that can occur in heat exchangers in the gaps between the tubes and tubesheet.

However, stainless steel does not provide corrosion resistance below the oxide layer. So why is this important? It means that stainless steel’s most prominent stainless property is corrosion resistance, which results from its ability to form and regenerate a chromium oxide layer in the presence of oxygen. This happens instantly, with formation speeds measured in nanoseconds and film thicknesses in microns. The chromium oxide layer forms on the stainless steel surface when chromium reacts with oxygen. Stainless steel comprises about 50 percent iron and anywhere between 10.5 percent (12 percent is a typical minimal amount) and 30 percent chromium, depending on the grade. Now contrast that with the formation of chromium oxide on stainless steel. A quick touch with a grinding wheel removes the rust and reveals the clean steel underneath. When exposed to moisture, the iron in carbon steel reacts with oxygen to slowly form a reddish iron oxide. What Is Stainless Steel?įirst, let’s look at carbon steel. The foundation for conquering many welding challenges starts with remembering that stainless steel acquires its stainless property by forming a layer of chromium oxide.

#CHROMIUM OXIDE LAYER HOW TO#

In the same way, fabricators should first think about what gives stainless steel its stainless properties and how to make sure those properties remain intact before, during, and after welding. I’ve gotta get inside this guy’s pelt and crawl around for a few days.” And, whenever possible, to look like one.

#CHROMIUM OXIDE LAYER MOVIE#

In the movie “Caddyshack,” Bill Murray explains his strategy for dealing with the gopher by saying, “In order to conquer the animal, I have to learn to think like an animal. This GMAW bead requires postweld cleaning to remove the heat tint (oxides) and spatter, both of which could promote corrosion.

Chromium oxide layer