The best welds are produced from clean, metal-to-metal contact without any foreign materials in the welding area. With that understanding, all materials, including stainless steel, should be cleaned and prepped before welding, including the process of removing any coatings that are used to prevent rust or oxidization.
Unfortunately, this first step is often overlooked, but, nonetheless, it is key in achieving a desirable weld. It is not the only crucial step, however. To realize the best possible welds on stainless steel, the cleaner the surface, the better.
Austenitic Grades
Austenitic stainless steels are the most common type used in fabrication stops. These stainless steels have a face-centered cubic crystal structure composed of iron, carbon, chromium and a minimum of 8 percent nickel. They are nonmagnetic and highly corrosion resistant due to their high chromium and nickel content.
Austenitic stainless steels cannot be hardened by heat treatment; instead, they are hardened by cold working. The high nickel content in austenitic stainless steels makes them suitable for low-temperature applications.
These base materials do not require preheating, however, they do have a maximum interpass temperature. This means that once the base metal reaches 350 degrees F, welding must come to a stop to let the material cool down. By utilizing a low-heat input process and focusing on welds that are convex, cracking may also be prevented. A flat or concave weld on these materials is more susceptible to cracking.
The two most common types of stainless steel – 304 and 316 – are austenitic grades. The reason behind their popularity is the ease at which they can be formed and welded, making them an ideal material for high-efficiency manufacturing.
In addition to the most common grades, there are many sub-groups of austenitic stainless steel with a wide range of carbon content. Alloying elements are often added, as well, including molybdenum, titanium and copper.
Austenitic stainless steels are often used in the production of kitchen sinks, ovens, chemical tanks, window frames and outdoor site furnishings. Outdoor furnishings include a range of items, such as benches and bollards, the short vertical posts used to control the flow of traffic.
Weld prep
It is crucial to clean any stainless steel, including austenitic, before welding, and the tools used to do so must be specific to stainless steel. This is due to the sensitivity of the material in the presence of any carbon steel. If traces of carbon steel become embedded in stainless steel, rusting occurs.
Similarly, extra caution is required when grinding carbon steel in close proximity to stainless steel. Carbon steel dust that is suspended in the air can land on the stainless steel, causing it to rust. Therefore, it’s wise to keep separate work areas for carbon steel and stainless steel.
Once the work areas have been delineated, stainless steel can be cleaned and must be done regularly to preserve corrosion resistance and maintain a shiny appearance. For stainless steel with minor surface stains, a relatively simple cleaning process of soap and water followed by a clean-water rinse is sufficient. A mild detergent or a diluted ammonia solution is also effective for cleaning when wiped down with a sponge, cloth or disposable wipe. An air blower can also be used for a quick drying option.
Alternate methods of cleaning, however, may be required for stainless steel that has become extremely dirty with traces of surface damage. When using harmful chemicals to clean stainless steel, follow the safety instructions provided with the cleaning agents. Also, be sure to use personal protective gear in accordance with occupational health and safety guidelines. It’s recommended to consider conducting a patch test by applying the cleaning solution to a discreet area of the surface to see the final finish.
Types of surface damage
Fingerprints: Use soap or detergent mixed with warm water to wipe down. Alternatively, commercial solutions, such as rubbing alcohol and acetone, can be used.
Oil and grease markings: Use acetone, rubbing alcohol or methylated spirits on the markings. Apply with a clean, non-scratching cloth several times until the oil or grease is removed.
Burnt-on grime: Soak in hot water and diluted ammonia or detergent, then remove the grime with a nylon brush. If needed, use a fine abrasive powder. However, use caution as abrasives will scratch polished surfaces. Rinse thoroughly with clean water.
Mild coffee and tea stains: Soak in hot water and sodium carbonate (washing soda) for tea stains and sodium bicarbonate (baking soda) for coffee stains. If the object is too large to soak, use a sponge or a soft cloth to clean.
Serious stains and oxidation: Use mild, non-abrasive cleansers. Apply with a soft cloth or sponge and rinse with clean water and dry. Cream detergents containing calcium carbonate or citric acid can also be used, but avoid using scouring pastes.
Heating stains: Use domestic metal polish to improve the appearance of tarnished stainless steel. Chrome polishes for automotive parts are widely available and extremely effective. Treat the entire stainless steel surface to avoid discolored patches.
Localized rust stains: Use a soft cloth to apply a solution of oxalic acid. Leave the solution on the surface for a few minutes to dissolve contaminating particles. Once clean, thoroughly rinse away all of the residual solution with water.
Cement, mortar and limescale damage: If mortar or cement comes into contact with stainless steel, rinse immediately. Use a 10 or 15 percent phosphoric acid-based solution in warm water. Spread cleaner evenly over the surface and wait for 30 to 60 min. Neutralize the acid with an alkaline cleaner or diluted ammonia and rinse with clean water. When dealing with limescale stains, dilute one part vinegar in three parts water and apply with a nylon brush. Specific products are also commercially available.
Graffiti: Use a biodegradable graffiti-cleaning spray or wipe. Avoid using knives or hard scraping tools as these may damage stainless steel surfaces.
Heavy damage: Heavily damaged products should be removed from service until a repair or replacement can be made. Heavy damage includes compromises to the structure, such as dents, cracks, breaks and rust. Remove rust as soon as possible to prevent worsening of damage.
Corroded surfaces, minor corrosion: Use an all-purpose lubricant to wipe affected stainless steel. Domestic cleaners with calcium carbonate or citric acid can be effective. Rinse thoroughly with clean water. If rust remains, treat as moderate. Treat for iron or carbon steel contamination if rust returns within a short time period.
Corroded surfaces, moderate corrosion: Use a phosphoric acid-based stainless steel cleaner. Identify damaged areas and spread cleaner evenly over the surface and let sit for 30 to 60 min. Then, use a spray-on alkaline cleaner to neutralize the acid. Wipe the surface clean with a paper towel and thoroughly rinse with water. If rust still remains, the stainless steel likely needs to be treated for severe corrosion or be replaced. If within a short period of time, the rust returns, treat for iron or carbon steel contamination (see the passivation section that follows).
Corroded surfaces, severe corrosion: A professional service provider is recommended due to the highly corrosive nature of serious rust treatments and the inherent risks to people and surrounding environments. Severe rust is treated with a pickling bath that contains highly corrosive hydrofluoric acid. After treatment, stainless steel can be passivized with mild nitric acid.
Passivation process
Re-damage of a clean stainless steel surface can occur through extreme heat and chemical exposure or mechanical means. These situations result in iron exposure, causing the material to once again be susceptible to rusting.
A regular passivation treatment is recommended to prevent rust and maximize the corrosion resistance of a stainless alloy. It is not a scale removal treatment, and ideally, the passive layer forms immediately after machining or passivation to completely cover the stainless steel surface. However, particles and contaminants can adhere to the surface and present the risk of reduced effectiveness of the original protective film.
For best possible corrosion resistance, a two-step passivation procedure is recommended:
First, thoroughly clean the part. Stainless steel cannot be passivated unless the surface is clean and free from contamination and scale. In serious cases, foreign matter can be removed through grinding, mechanical abrasion or pickling involving a nitric/hydrofluoric acid mixture.
Second, apply the passivating treatment. Here, the part is immersed in a passivating acid bath using three different approaches: nitric acid, nitric acid with sodium dichromate or citric acid passivation. The treatment depends on the prescribed acceptance criteria as well as the chrome content and machinability characteristics of the grade.
When general care for stainless steel is combined with a thorough understanding of how to prepare its surface, welders will achieve the best welds possible during the first pass. Often, the time committed to surface prep up front will result in saved time overall.
