What is Weld Porosity or Welding Porosity? Definition, Types, Causes, How to Prevent & Fix it?

What is weld porosity or welding porosity? When there are gaps or cavities between welded metal surfaces, this is called welding porosity. The gaps are empty spaces where the metal hasn’t melted completely, which can cause problems later on. In order to prevent this from happening, you can do a number of things.

Unlike cracks, undercuts, overlaps, and lack of fusion, porosity is not detrimental to structural integrity. Unfortunately, it is unsightly and becomes an eyesore when painted. Porosity does not propagate like a crack because it does not have sharp ends. Welding effectiveness is decreased when porosity is limited per structural codes. The goal should always be to achieve zero porosity, regardless of these limitations.

In the presence of porosity, the strength and integrity of a weld can be severely compromised, which means that a porosity-contaminated weld is likely to fail most weld tests and examinations.

Define Welding Porosity

Porosity is a type of welding defect that occurs when external gases, most commonly oxygen, are trapped within a welded joint. It is possible for external gases to enter a weld and become trapped internally, resulting in cavities, or it can be forced out of the weld, leaving holes and pits on its surface.

The release of dissolved gases in the weld metal causes welding porosity, a metallurgical discontinuity that appears as pores during solidification. Welding porosity can be either a single or a group of pores. A welded part may have a single porous area, a local cluster of pores, or a scattering of porosities.

Workflow of Welding Porosity

Poor gas shielding usually causes nitrogen and oxygen to absorb in the weld pool. Random porosity occurs when pores are oriented in random directions at uneven distances. Separated from all sides by an inch is also an option.

As a result of the absorption of nitrogen, oxygen, and hydrogen gases trapped in the molten weld puddle and released during solidification, weld porosity occurs, which creates pockets or pores on the weld surface. There can be porosity on the surface of a weld or within the weld bead.

An elongated cavity result when entrapped gas attempts to escape. As opposed to pinholes, piping porosity appears funnel-shaped from top to bottom, rather than spherical.

Causes of Porosity in Welding

Porosity can appear in all forms of welding processes and positions with the direct cause of the occurrence varying from one situation to another. Below are some of the most likely causes for porosity and specific instances which will only occur with certain welding procedures.


When x-rayed, they appear like a bone joint, giving them the name Herringbone Porosity. During the molten state of the weld metal, wormholes are formed by the unwanted gas trying to escape through the elongated pores. Due to their external visibility, they are generally easy to identify.


The formation of excessive gas will occur when there is gross surface contamination or when the paint or primer coating is very thick. In horizontal-vertical T joints, the vertical member of which is fillet welded on both sides, entrapment is more likely to happen in crevices.

Over Spraying can result in the coating thickness on the edge of the vertical member exceeding the manufacturer’s recommended maximum, typically 20mm.

Distributed Porosity & Surface Pores

Since Distributed Porosity and Surface Pores have similar characteristics, they are placed together. The most common form of porosity, distributed porosity is characterized by different types of porosity.

A weld with distributed porosity will usually be detected by imaging the weld bead with x-rays since it has many fine pores trapped inside it.

Pores on the surface are visible to the naked eye and easily identifiable by their large, numerous holes. Their presence is often accompanied by Internal Distributed Porosity, which makes them a severe defect.


The weld bead normally exhibits distributed porosity in the form of fine pores. Porosity distributed throughout the surface usually indicates a large amount of surface-breaking pores. Weld metal becomes trapped in this state after nitrogen, oxygen, and hydrogen are absorbed in the molten pool and released upon solidification.

Weld pools usually absorb nitrogen and oxygen due to poor gas shielding. In the shielding gas, 1% air entrainment will cause distributed porosity, and more than 1.5% will result in gross surface-breaking pores. Porosity may be caused by leaks in the gas line, high gas flow rates, draughts, and excessive turbulence in the weld pool.  

In addition to moisture from inadequately dried electrodes, fluxes, and the workpiece surface, hydrogen can also come from the work-piece itself. Hydrogen can also be found on the surface of a work-piece or filler wire due to grease and oil.

Welding can generate copious amounts of fumes from surface coatings like primer paints and surface treatments like zinc coatings. As a result of fillet welding on both sides, T joints will be more prone to trapping the evolved gas than butt joints.

Weld-able primers deserve special attention. When using welding porosity processes, the thickness of the primers should not need to be removed, but if they exceed the manufacturer’s recommendations, porosity could result.

Crater Pipes

It is considered a very minor form of porosity since it can be easily detected with the naked eye and repaired without any trouble. In the final solidification process, crater pipes appear as tiny pinprick-holes at the end of weld runs. In spite of being visually displeasing, these holes and Crater Pipes rarely affect the structural integrity of the weld.

By setting your machine’s down-slope correctly or by allowing the shielding gas to flow over the cooling weld a few seconds longer, these problems can be avoided.


It is often associated with gas porosity and forms during weld pool solidification.

In addition to crater formation, autogenously welding techniques will promote pipe imperfections by stopping the welding wire from entering the weld pool before extinguishing the arc.

Weld pool solidification leads to this imperfection due to shrinkage. Therefore, conditions that cause the liquid to solid volume change to increase will promote the formation of solids. A weld pool that has been extinguished will rapidly solidify.


The mornings in many countries are cold and damp; moisture can cause many welding defects, as well as porosity. When you weld over a weld joint with moisture or dew or even condensation, the weld pool’s temperature can drop rapidly and prevent it from solidifying normally. As a result, the weld has Distributed Porosity.

When using a Stick-Welder, the flux-coated electrodes can easily absorb moisture, and this can contribute to porosity.

Inclusion Porosity

As a result of porosity, isolated pockets are present within the weld metal, causing the metal to be porous. It is usually caused by dirt or foreign objects getting trapped in the welding joint as a result of getting trapped in joint.

Arc Blow Porosity

In TIG welding, it can cause serious problems. This phenomenon occurs when the arc moves across a weld puddle, creating a draft that draws gas pockets into the joint.

TIG welding can result in an arc blow. A draft is created when the arc moves across the weld puddle, sucking gas pockets into the weld. If possible, this should be avoided as it can lead to serious problems with the weld.


A weld joint can become porosities if it contains slag or other foreign objects. Weld metal may contain air pockets if these objects are included. Porosity is created when these air pockets in the weld become gas pockets.

Atmospheric Condition

There are three main factors that contribute to the porosity found in all types of welds. It refers to the quality of the atmosphere surrounding the pool of weld. Weld pools do not receive enough shielding gas or too much shielding gas is being applied.

Bad Welding Technique

Porosity can be caused by below-standard welding techniques and procedures regardless of your skill level. The causes of this problem include using MIG welding at an excessive angle, using old or contaminated filler wire materials, using too much anti-spatter spray, or welding an open joint with a large root.

Contamination of Cylinder

Porosity can also be caused by cylinder contamination or age, even though these factors are unlikely. In order to avoid contamination of both the cylinder and the gas, cylinders should be hired from a professional supplier.

How to Prevent Porosity Weld?

Porosity will always be a possibility in your welds, regardless of how you prevent it. Certain circumstances can make porosity disappear.

Adjust Moisture

Whenever moisture is spotted in your weld joint, simply soak up the excess moisture with a clean rag on a cold, damp morning. It is possible to quickly heat the joint and ensure that all moisture is evaporating if you are particularly concerned.

A standard procedure is to store Stick-Welding Electrodes in a dry location. Electrodes are often baked in specialist ovens to ensure evaporation of moisture is complete, ensuring the electrode’s efficiency.

Adjust your Gas Flow Rate

A high flow rate for wire processing can actually lead to porosity. It is not always better to have more flow.  When the shielding gas is flowed at high rates, it creates turbulent flow, which can actually introduce air into the weld. It is possible to have other problems as well, such as wind, reach problems, or gas leaks in your gun assembly if you need to increase your flow rate beyond 60 in order to avoid porosity.

When using a shielding gas, make sure your flow is appropriate for your application. Wire processes typically require 30 – 50cfh. Due to the difference in cup sizes, there is more variance, but generally 15-25cfh is ideal.

Vacuum Chamber Welding

Welding in a vacuum chamber can completely eliminate porosity. By doing so, you will ensure that the welding material is not contaminated and that the weld joint is free of foreign matter.

Wrong Welding Electrodes

It is also possible for porosity in welds to be caused by the use of incorrect welding electrodes. The reason for this is that some welding electrodes are more likely to form porosity than others. Porosity can also be formed if the welding electrodes are not properly matched to the metal being welded.

Keep an Eye on Travel Angle

Whether you are pulling or pushing, keep it at 10-20 degrees from perpendicular. Porosity, poor penetration, and excessive spatter can result from an excessive push or pull angles.

Porosity is a bigger issue when welding aluminum than when welding carbon steel or stainless steel. Aluminum is more affected by everything that causes porosity on steel. Due to the presence of deoxidizers, carbon steel wires can withstand higher levels of moisture, rust, paint, oils, etc., while aluminum wires cannot.

Welding Surface Cleaning

Porosity is significantly increased by contaminants such as moisture, grease, oil, cutting fluids, rust, paint, and other sources of hydrocarbons. Galvanized steel, for example, presents a problem because of its surface coating. Before welding, you should grind off the zinc coating. 

  • Make sure the work-piece surfaces at and near the weld location are clean
  • Remove any contamination from the surface, including oil, grease, rust, and residue from any welding operations
  • To expose bright material in the joint area, remove any surface coatings
  • Make sure the primer thickness is below the manufacturer’s recommended level


Introducing dirt or foreign objects into the weld joint can cause porosity in welds. The weld may become weaker and more susceptible to failure as a result. Additionally, it can make the weld less aesthetically pleasing.

Keep an eye on Gas Leak

You can ensure that all connections are tight & make sure the hoses are not damaged. In some wire feeders, an improperly seated gun back end can cause a leak that is hard to detect. As far as it will go, make sure it is pushed back. The o-rings on the power pin should also be checked for damage.  Leaks of shielding gas can occur even if the o-ring has a small nick.

Porosity can also result from a faulty gas hose or gas cylinder; test for leakage by spraying a mixture of water and alcohol-free soap on the lines and top of the cylinder. If bubbles appear, you have a leak.

Avoid Drafting

When a bay door is open or a fan runs, enough shielding gas is blown away to create porosity.  Position your body between the fan and the weld if you cannot stop the draft or need the fan due to the heat. The welding fumes will also be blown away from your breathing zone as a result.

It is also possible for poor welding posture to cause porosity in welds. Due to the bad position the welder may be in, gas pockets can form in the weld puddle. The quality of the weld can also be compromised as a result.

Clean Environment

Cleaning the welding environment is another way to prevent porosity. If possible, avoid welding in dirty or dusty environments and live in a damp environments.

Dusty Environment

The formation of porosity in welds can also be caused by welding in dirty or dusty environments. Because dirt and dust can get trapped in a welding joint and create pockets of air, the pockets of air will later become gas pockets.

Humid Environment

The formation of porosity in welds can also be caused by welding in humid or wet conditions. As a consequence of the humidity, the welding material can become damp, causing gas pockets to form. Weld joints can also rust, which weakens them.

How to Fix Welding Porosity?

Porosity must first be detected by a magnetic particle inspection technique, such as radiography or ultrasonic inspection. Pores in thicker sections are difficult to detect because they are so small.

It is impossible to fix the porosity itself. It is better to remove the localized gouging or grind it out in order to fix the weld. However, porosity must be contained in order for this to be possible. The weld must be removed, the joint must be prepared again, and the joint must be welded as described in the welding procedure if it is widespread.

  • It is possible to fix porosity by removing the weld and rewelding it.
  • Slag inclusions can cause porosity, which can be repaired by removing the inclusion.
  • Porosity can also be fixed by patching the weld with a plate.
  • Make sure the weld is free of porosity by retesting it.
  • It can be fixed by welding over the porosity to fill it in.
  • Hide the affected area by painting or sealing the weld.
  • A weld filler rod can be used to fill the porosity.
  • It is also possible to remove the porosity of the weld by grinding it.

Types of Welding Porosity

Cluster Porosity

Welding pores that are concentrated locally are known as cluster welding porosity. Cluster porosity refers to a group of pores separated by a considerable distance from a porosity-free weld.

When a welder starts or stops welding, such clusters are often associated with changes in welding conditions. As long as shielding gas is absent in TIG welding, disturbed shielding gas supply results in welding porosity similar to cluster porosity.


In welding, cluster porosity occurs during welding start-up and shutdown, so it is associated with poor welding practices. Cluster porosity can also occur when shielding gas is blocked or stick welding rod coating is defective during the affected period.


By using good practices, cluster porosity can be controlled. To ensure good welding initiation and termination, more emphasis should be placed on welder training. You can inspect and maintain the coating on stick welding rods. Be sure that there is a continuous supply of shielding gas without any blockages or damage during welding.

Welding Piping Porosity

It is possible for elongated or tubular gas cavities to form during welding in some rarer cases. The porosity is referred to as piping porosity or wormhole porosity. A piping porosity is a type of welding porosity that has a length greater than its width.

Perpendicular to the weld, piping porosity is oriented. Fillet welds have porosity that extends from the weld root to the face side. One or two visible piping pores on the surface indicate that the weld surface has many piping porosities.


In welding piping porosity, rapid solidification is the main cause of wormholes and porosity.


In welding processes involving high arc energy, such as electro slag welding, this is a common problem. Controlling the depth-to-width ratio or using alternate techniques is necessary to prevent piping porosity.

Uniform Scattered Porosity

Rather than being concentrated in a small area, scattered porosity refers to the distribution of welding pores over the entire length of the weld. It is possible for them to be present throughout a multi-pass weld.


  • A welding technique that disables gas shielding on a weld pool, such as using a long arc length.
  • Windy conditions can make welding difficult.
  • Use of the wrong shielding gas using nitrogen for welding carbon steel or low alloy steel materials or contamination in the shielding gas.
  • Welding surfaces are contaminated with moisture, rust, paint, grease, or coating.
  • Materials with defects include base metals with high levels of lead or other low melting elements.


  • Ensure that the welding technique is correct. If the wind is blowing, you should never weld with a high arc length.
  • Make sure the weld surface is clean before starting the welding process.
  • Make sure you’re welding on the right base metal.
  • Make sure welding shielding gas is supplied in accordance with AWS gas specifications.

Linear Porosity

Observed as multiple pores in a straight line, linear welding porosity or aligned porosity is a type of welding porosity. Often, linear welding porosity is regarded as a sign of inadequate joint penetration in the root pass of welds.

A weld with defects that line up is bad, and a weld with defects at the root is even worse. When the welded component is heavily stressed during service, the lined-up pores can get connected very quickly by short cracks to become one big catastrophic failure.

There are two types of linear porosity: elongated pores and round pores. It can appear in the weld root, the weld surface, or even between the weld beads themselves.


The cause of linear or aligned porosity is the release of gases from any contamination, such as moisture, grease, paint, or rust.


The welding area should be cleaned before welding begins. To remove moisture or contamination, apply preheat. Coated or painted surfaces should not be welded.

Define Welding Gas Porosity

There is a phenomenon known as gas porosity in molten metal’s, as it can hold a very large amount of gas that has been dissolved, unlike solid metals. This causes gas bubbles to form in the material as it cools, as a result of the cooling process. It is also possible for gasses to be introduced during the casting process due to turbulence. There is a risk that this will occur if the molten metal is poured into the mold too quickly.

Porosity Increment in Weld

Even as little as 1% of air entrainment in the shielding gas can result in distributed porosity, whereas greater than 1.5% will cause gross surface breaking pores in the shielding gas. The most common causes of porosity in weld pools are leaks in the gas line, too high a gas flow rate, draughts in the pool, and excessive turbulence in the pool during welding.  

Gas line leaks, excessive gas flow rates, draughts, and turbulence in the weld pool are all common causes of porosity. In addition to inadequately dried electrodes, fluxes, and the surface of the work-piece, hydrogen can come from a number of sources.  

Tolerance Level of Weld Porosity

On or inside your welds, there should be no porosity. When a structure or product has porosity, it becomes weak. There are not many weld inspectors who will not accept even the tiniest Crater Pipe in a 10-meter weld; nor will any decent weld school examination accept it either.

A definitive answer is not possible because porosity will vary depending on the application. A weld’s porosity should be less than one percent. The quality of the weld can be seriously compromised by anything more than this, so it is best to avoid it if at all possible.

Define Undercut in Welding?

A bad welding technique usually causes undercuts rather than the porosity. There can be sharp edges or cracks along the edges or below the joint if the filler material fails to completely fill up the joint space. Weld examiners consider it a defect, but it can easily be fixed.

Define Blow Hole

It is sometimes possible for entrapped gas to form a large cavity during welding. This is called a blowhole. It is possible for a single pore to be open to the surface if a subsurface defect exists.

Welding Porosity Relationship in Shielding Glasses

The characteristics of welding shielding gases differ according to the type of gas used. As an example, nitrogen will not cause any problems when used for shielding or purging austenitic stainless steel welds. Nitrogen will cause welding porosity when used for carbon steel or low alloy steels.

Aluminum welding is also prone to porosity due to hydrogen. Most materials, including carbon steel, stainless steel, aluminum, and copper alloys, are porous when welded with hydrogen, but nickel-base alloys are not.

Welding porosity can be caused by nitrogen in carbon steel, low alloy steels, nickel-base alloys, and copper alloys.

Porosity in Metal

In additively manufactured metal parts, porosity refers to whether the layers are filled with cavities or holes. Some hard-surface materials, such as stainless steel, solid surface, and some rigid plastics, are nonporous, but laminates, granite, and some plastic materials are porous.  

Lotus-type porous metals or lotus metals are porous metals with long directional pores. Because of the gas-solubility difference between liquids and solids in lotus metals, pores form from gas molecules insoluble in the solid.

Wrapping it Up

You can prevent most porosity in welding by understanding its causes. A properly set-up machine, adequate gas coverage, and clean material are half the battle won. Stopping welding to check whether it’s turning out alright is far more efficient and less frustrating than grinding out 16 inches of bad weld, especially if you’re in a tight spot. You can establish a baseline by knowing how a good weld feels. When something feels off, you’ll be able to tell.

A welder is constantly seeking ways to reduce porosity in the welding process in order to reduce defects. The porosity of thinner walls tends to be lower, while the porosity of thicker walls tends to be higher. It is best to design your part with uniform walls so that the metal can flow into more of the part and prevent porosity better than if the walls are thin and thick.  

FAQs on Weld Porosity

Is it ok to weld over porosity?

Occasionally, but not always the crater Pipe can be welded over the hole since it is not contamination, but malpractice.
In all other cases of porosity, your weld bead is very likely to continue to be contaminated by welding over the problem. In the best-case scenario, you will be able to cover it; however, the internal damage will remain unrepaired.
Furthermore, welding over porosity will produce an unattractive weld that is easy to identify. It is important to repair porosity correctly.

Can you stop Porosity Permanently?

A variety of factors can contribute to porosity, so there is no definitive answer to this question. Porosity can be reduced in a weld by many of the methods listed above. You can achieve an impermeable weld by using these methods.

Can you burn out Porosity?

There is no burn out of pores as they travel from weld pass and never leave the original weld pass. A carbon arc gouging or grinding must be used to remove it.

Can you use paint to stop Porosity in weld?

The affected area can be hidden by painting or sealing the weld. Using this method, porosity in a weld can be fixed relatively easily. The paint or sealant will eventually wear off, so this is not a permanent fix.


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