When stainless steel gets welded, its surface layer gets affected a lot. Heat tint appears on the layer, which can diminish the appearance of the material. So, we have to clean stainless steel after welding so we can use it. There are many ways available to do this job, but the best methods fall in the category of electropolishing. Electropolishing not only makes the object look good but also increases its lifetime. Thus, whenever you weld steel, you have to clean its layer before reusing it.
List of Contents
- Common Faults that Appear after Welding Stainless Steel
- Methods of Surface Cleaning Stainless Steel after the Weld
- 3.Electrochemical Methods
- Frequently Asked Questions FAQs
Common Faults that Appear after Welding Stainless Steel
Stainless steel has a layer of chromium oxide over its layer. When this layer comes in contact with heat, it becomes thicker and results in discoloration, changing the color of the surface layer. These phenomena mostly occur within the heat-affected zone (HAZ) due to the applications of light interference, which states that all the colors we see are the result of the reflection of light from the surface of a material.
Different thickness of surface layer gives different colors. The temperature of your weld is also a primary factor in producing these colors over the workpiece—the more the temperature of your weld, the darker the color of your weld.
The look of the stainless steel does not remain appealing after the introduction of these colors. This behavior of the steel layer is called the rainbow effect in industries. Proper welding techniques without exposure to excessive heat and better shielding methods can prevent these heat tints on stainless steel.
2.Risks of Corrosion
Stainless steel has a layer of chromium oxide over its surface to prevent it from rusting. But applying heat for a long time distorts this layer. We cannot restore The passive state of stainless steel without removing these destroyed oxide layers. The damaging of this oxide layer greatly reduce the durability and strength of the unit. Thus, welds not only blur the appearance but also affect the power of the material.
To prevent the destruction of the surface layer, a welder can use balanced alloy composition to avoid precipitation. Molten gas should be adequately shielded from reactive gases, and choosing welding parameters appropriately will reduce the risk of depletion of the surface layer. But in most scenarios, you have to apply surface polishing to make the coating look good after the weld.
Methods of Surface Cleaning Stainless Steel after the Weld
From the above discussion, it is clear that steel needs to get cleaned after the welding job. There are many methods available for making the surface of steel look good. Mechanical and chemical methods were used in old times, but now the advent of electropolishing has dramatically improved steel welds’ cleaning.
Steel can be polished to make it look new. If the tints are lighter in color, then they don’t hesitate to go away. But to get rid of dark stains, you have to use the right electrochemical methods to restore the passiveness in the material.
Mechanical cleaning is commonly used in small works because of its low cost and satisfactory results.
Abrasive brushes are used to clean the weld’s surface to remove the welding gas oxides and Some of the steel layers below these oxides. The abrasives used must not be more rigid than the layer of steel in order to save the layer from roughing out instead of plaining down.
The abrasives must have decisive cutting actions and brightening actions. They should also be able to have lubricating action so that they can give a subtle finish to the material.A welder should use Grains of different sizes to remove the roughness of different levels.
There are three steps of proper mechanical cleaning of a steel weld. Firstly, you should make the weld surface rough so that abrasive grains can catch it easily. Afterward, grains are thrown over it, and they are rubbed to level down the layer. Finally, you should mix the grains with lubricant in order to stimulate the corrosion process.
In Grit blasting technique, we fire particles over the surface using a nozzle using high pressurized air. This method is widely used in cleaning the abruptness in the surface of steel after weld. Blasting can remove high temperature and dark tints, but you have to ensure that sand (probably of olivine type) used in blasting must not be contaminated or used before.
Otherwise, steel will get polluted instead of being cleaned using blasting. The surface roughness is the limiting factor for these methods. Using low pressure and a small angle of approach, a pleasing appearance can be achieved for most applications.
Results of using Mechanical Methods
The use of mechanical methods doesn’t form a compact and thick outer layer over the steel surface. Thus, stainless steel can get corrosion when subjected to highly corrosive environments. Mechanical methods also take so much time and energy. Beginner and hobbyist can use these methods of mechanical cleaning to preserve the steel welds after discoloration.
Chemical methods can remove dark tints with ease and give excellent surface finishing. Welders get the most delicate welded surfaces by using electropolishing after applying chemical techniques. Thus, for using chemical methods, the surface of the metal must be cleaned thoroughly because a chemical method is contaminated easily.
All the organic contaminants like grease, oils, and paints must be removed from the workpiece’s surface. This can be done using a product known as Avesta Cleanser (containing phosphoric acid), which is sprayed over the surface to be cleaned using chemical methods. Using Avesta cleans also remove the rusts on the body and brightens the surface. After use, you must rinse the layer with water before submitted to chemical methods.
Pickling is the most common chemical method to remove oxides and rusted materials. Pickling usually involves the spray of acidic solutions like nitric acid or hydrofluoric acid over the welded part. Thorough rinsing with clean water having chlorine to some percentage must be done after applying to pickle.
The effectiveness of pickling methods depends on the strength of the acid you are using and the surface’s requirement. Contaminants in the welded body also reduce the results of pickling to a great deed. The temperature should also be appropriately operated while using pickling as the reaction rate of acids increase by increasing the temperature.
1.Pickling is a Bath
The most common technique of pickling involves the sinking of the workpiece thoroughly in an acidic solution. But this method usually requires suitable equipment, which becomes challenging to find if the working surface is large enough.
User should keep the bathing temperature between 25 to 65 degrees centigrade, depending on the type of metal. Over-pickling is more dangerous than incomplete pickling as it gives a rough surface that can not be corrected again. The effectiveness of bathe pickling is determined by the acid concentration and temperature and depends on the metal (mainly iron) content in the bath. Increased iron content requires a high bath temperature.
2.Pickling with pickling Paste
Pickling paste consists of an acidic mixture added with a binding agent. This method is suitable for limited pickling areas like only the welded zone. The paste is applied to the welded part using an acid-resistant brush. At low temperatures, the paste is un-affective; thus, you have to use high temperatures, preferably more than 30 degrees.
A greater risk is that of the paste drying out due to evaporation, resulting in reduced pickling effect and rinsing difficulties. We should rinse the surface with water before the chemical dries. Even if you are going to neutralize the pickling paste on the metal surface for environmental and practical reasons, thorough rinsing with water is vital.
3.Pickling with Pickling Solution
Pickling solution usually consists of a mixture of nitric acid and hydrofluoric acid added with a binding agent and specific surface-active agents like ammonium salt to gain good thixotropy (Good reaction against applied stress) and the right viscosity. It is suitable for pickling large surfaces, e.g., when you desire the removal of iron contamination.
Results of Using Chemical Methods
Chemical methods achieve better results than mechanical techniques; however, they are also very dangerous to use. Only certified operators use these methods with proper protection. Exposure to acid can result in skin burns and even skin damages.
This is why chemical methods are usually carried out by specific automated machinery without the intervening of humans. Once done correctly, these techniques give outstanding outcomes and can be used on commercial and professional levels for showing gleaming results.
Electrochemical Methods are the best ones when it comes to passivating the real properties of stainless steel. When it comes to electro cleaning, you don’t have to deal with the annoying problems of dirt, buffing compounds, or excessive noise.
In addition to that, these methods don’t change the appearance of the steel surface at all: yet simply restore it to its original state. Electrochemical cleaning is also, in a way, a very flexible method of cleaning, as it can be carried out on the site during installation. Many fabricators perform this method as a remedial treatment.
Electrochemical Cleaning Technique
The process is straightforward and commonly used on the professional level for giving blazing results. The method includes dipping the welded part in an electrolytic solution and passing the electric current through it. This process depends on the principle of electrolytic reaction, which results in the dissolving of surface rust, iron contaminations, and all kinds of blots and blemishes. As a result of this, the stainless steel is left surgically clean.
If the workpiece is large, which cannot be submerged in a solution, then the solution is applied over the metal using a brush, and electricity is passed through the metal, creating a shining surface. The current user may be AC or DC; however, the type of currently used type seriously affects the final results.
In addition, electrochemical cleaning ensures that there is no further build-up of contaminants that may facilitate corrosion. With electrochemical cleaning, there is also a significant decrease in the overall pitting corrosion of stainless steel.
Results of Using Electrochemical methods
Electrochemical methods are elementary to give and generate ideal results. On the other hand, unlike chemical processes, they are also intrinsically safer to adopt. They take much lesser time than other methods and restore the passiveness of stainless steel a significant bit. Electrochemical methods erase the previous defects in the material and increase its resistance against corrosion in future applications.
Choice of Method
Choosing a method from all of the mentioned above is essential and determines your materials’ passiveness. The choice of method depends on corrosion resistance, hygienic considerations (pharmaceuticals, food), or whether the visual appearance is the sole criterion.
Provided that the surface roughness so permits, both mechanical and chemical methods can be used. If you require a magnificent appearance, then you should use electrochemical methods. Electrochemical methods also save time and are easier to give as compared to mechanical and chemical processes.
Frequently Asked Questions FAQs
How do you remove discoloration from stainless steel?
A worker can cure discoloration by using certain agents like vinegar or club soda. Use a soft cloth to wipe against the surface of the stainless steel. Rinse with warm water afterward and dry it out. Finish it off using polish on the stainless steel to give a gleaming finish.
How do you clean after welding?
Vinegar can be acidic at high temperatures and can cause corrosion. When using it to clean metals, you must be cautious and rinse the surface thoroughly with water. Leftover residues of vinegar on the metal surface can result in damage to the layer.
Does vinegar ruin stainless steel?
There are different ways that you can adopt to clean after welding. Chemical methods are most frequently used, which involves the pickling of metal surface in an acidic solution or pasting certain chemicals on it. Electropolishing is also used, which can restore the passivity of stainless steel very efficiently. The type of method chosen usually depends on your applications and required output results.
This article discusses all the essential methods that the welding community uses for stainless steel cleaning all around the world. Each method has its benefits and conditions for usage. Choosing your strategy depends on the workpiece’s strength and formation you are working with. Welders usually prefer chemical and Electrochemical methods for achieving shining results at a reduced cost and time.