Skip to main content

«  View All Posts

What is filiform corrosion? How can you prevent it?

Thomas Besley | 6 min. read

What is Filiform Corrosion?

The vast majority of the general public will have come across filiform corrosion at some point. Whether they will have been able to identify it as filiform corrosion is another topic.

The point remains, filiform corrosion is commonplace. Perhaps the industry that is most affected by filiform corrosion is the transportation industry.

Some vehicles or trains end up corroding a lot more than others. It can be difficult to know why it happens and what is going on to speed up the process.

As adhesive specialists here at Forgeway, we specialise in telling people what they can do with glue. The transportation industry in particular asks us how we can help with their filiform corrosion issues.

This article will go over what filiform corrosion is and how you can prevent it from happening. The article will focus on adhesive’s uses in the transportation industry as an example. However, the methods and tips we discuss will apply anywhere filiform corrosion can take place.

Adhesives can form part of the solution, but there are multiple solutions you can choose from. By the end of this article, you will be able to determine what solutions you can implement to help prevent filiform corrosion from taking place.

What is filiform corrosion?

We included this section with the intention to provide some background on what filiform corrosion actually is. This will then allow you to determine why it could take place in your situation.

Filiform corrosion is a type of corrosion that looks like little worms or filaments spreading under the coating on a metal. It can only occur when there is a layer or coating (usually paint) over the top of the metal. 

Filiform corrosion on a painted metal surface
Credits to Matador, CC BY-SA 4.0 Link to Credits, via Wikimedia Commons

Filiform corrosion will not affect metals that have no coating.

It only occurs in certain metals too. Aluminium and steel are the two metals that are commonly affected by filiform corrosion. It can also affect iron and magnesium. Although aluminium and steel are the most prone, particularly as they are the most commonly used in the transportation industry.

The reason it can only affect metals that have a coating on top is simple. If there are any imperfections or inconsistencies in the coating, water can get in between the coating and the metal surface causing the corrosion process to start.

The actual scientific cause is quite complicated and very technical. But, in simple terms, filiform corrosion can only take place when there are imperfections in the coating to allow the water to get through.

Damage and poor preparation are the main causes of imperfections in the layer of coating. Poor preparation can happen when cutting, shearing, or machining the metal.

If there is no coating or there are no imperfections, filiform corrosion cannot take place.

What affects filiform corrosion?

As discussed above, water is the main cause of filiform corrosion. But there are factors that affect how that water gets there.

There are two main groups these factors fall into, controllable factors and uncontrollable factors.

We will use the transportation industry as an example in this section as this is where these factors commonly occur.

Factors you can control

You will be able to control manageable factors, as the name suggests. These factors originate from how the vehicle or structure was initially created. They stem from the manufacturing process.

Improper preparation is the main factor that can ultimately lead to filiform corrosion. The meaning of improper preparation may be confusing, so here is an example. 

A bus manufacturer was installing some panels on the side of a bus. The operative marked where he needed to place the panels.

Poor surface preparation will result uneven edges and potentially filiform corrosion
Operative marked where he needed to place the panels

There is a problem. The operative had marked this by creating gauges in the metal plate.

Blue lines show where the metal has been gauged
Gauges highlighted in blue

You may be asking, “won’t they use paint to cover the gauges?”

It’s a very good point, the manufacturer will paint over the metal. But it won’t solve the problem.

The gauges you can see will cause the paint to be inconsistent and there will likely be burrs as a result of them. These burrs can, and probably will snap off at some point.

Where the burr once was, will likely result in the opportunity for the water to creep beneath the paint as the paint no longer covers it. This then will increase the chances for filiform corrosion to take place.

Other manageable factors include using a joining method that increases the chances of filiform corrosion. These joining methods are typically where the joint design allows the water to ‘pool up’. Once it has gathered, the water can find cracks through the paint and begin the filiform corrosion process.

Factors out of your control

As described, manageable factors are controllable. On the other hand, external factors will nearly always be out of your control.

External factors are mostly a problem when end-user of the bus (sticking with the bus example) is in a problematic climate.

“What even is a problematic climate?” you might be wondering.

A problematic climate is by no means an official term, although it is quite self-explanatory. Any climate that could pose a problem and potentially cause filiform corrosion we would call ‘a problematic climate’.

Hot and humid climates are renowned for being problematic. The same could be said of climates with extended periods of cold.

They both present problems for different reasons though. Moisture is always the root cause of the problem. But how that moisture gets there is why these climates differ.

In a hot and humid climate, there is always plenty of moisture. The water will ‘pool’ in the parts of the bus where the manufacturer’s preparation was insufficient. This pool of water will then find its way into cracks in the coating and cause the corrosion process to start.

In colder climates, it’s not so much the amount of moisture but more about the moisture’s contents.

During the colder months, roads are covered in salt. The salt then flicks up into the bus (still using the bus example) and sticks to the metal surface. It is a massively problematic external factor.

Gritter spreading salt which can cause chips in paint leading to filiform corrosion

Salt can speed up the filiform corrosion process by up to five times.

The salt problem isn’t confined to cold climates though, coastal roads also face the same problem. The increased amounts of salt and sand in the air create the same problems as salt-covered roads in cold climates.

External factors aren’t limited by the types of climate. The other significant factor is damage to the coating.

Whether unintentional (stones flicked up) or otherwise (vandalism), the coating can get damaged. This then obviously allows the water to get in between the coating and the metal and the corrosion process to start.

Top tips to prevent filiform corrosion

By now you should know what filiform corrosion is and what causes it.

If you are worried that your product meets some of the criteria above and is susceptible to filiform corrosion, don’t worry. There are several methods that can reduce the likelihood of it taking place:

Tips to prevent environmental factors causing filiform corrosion

  1. Regular checking for damage
  2. Washing salt off the metal
  3. Reducing the risk of damage (installing mudguards etc)

mud guards on vehicles can prevent filiform corrosion
Mud guard on a bus will help reduce filiform corrosion

These are all methods to mitigate the risk of external factors though, so what about controllable factors?

Obviously, if they are controllable, you can control them. But how do you ensure you are getting it right?

There are several things you can do to mitigate the risk posed by manageable factors:

Tips to prevent controllable factors causing filiform corrosion

  1. Ensure the coating has covered the entire surface of the metal
  2. Check for any imperfections in the metal that could impact the effectiveness of the coating
  3. Ensure the edges have been deburred

Deburring the edge of metal to reduce the risk of filiform corrosion
Deburring the edge of metal to reduce the risk of filiform corrosion

The last point is the most significant manageable factor by far. If you haven’t deburred the edges, the metal will be much more susceptible to filiform corrosion.

But what do we mean by deburring the edges?

When metals are cut into shape, the edges will usually be square and ‘burred’. Deburring will round off that squared edge to make it more rounded.

Burred edges on metal
Typical examples of burred edges

If the edges have been deburred, the coating will evenly spread across the surface. However, when the edges aren’t deburred, the coating will not spread evenly and will be thinnest at the edges.

metal edges that have been effectively prepared
Deburred edges of metal

A thinner coating results in a metal that is much more susceptible to filiform corrosion.

How can adhesives reduce filiform corrosion?

Adhesives themselves will rarely be able to stop filiform corrosion from taking place. However, they can play a significant role in reducing the chances of it happening.

Some adhesive manufacturers include anti-corrosive additives in their products. This then gives the adhesive the ability to slow the corrosion process.

But this will not be the saviour to the filiform corrosion problem. Anti-corrosive adhesives will not protect a poorly prepared metal surface from corroding.

There are other reasons that the transportation industry (and others) use adhesives to help prevent the metal surface from corroding.

The age-old debate around mechanical fasteners v adhesives is particularly relevant in this situation.

mechanical fastener on sandwich panel
Mechanical fasteners in the side panel of a caravan

This is because mechanical fasteners can actually increase the chances of filiform corrosion taking place. You will need to install the mechanical fastener before the coating is applied.

In a similar issue to the burred edges, the coating around the mechanical fastener will not be as thick. This then leads to an increased chance of water seeping in.

Mechanical fasteners also cause the metal to vibrate. The vibrations increase the likelihood of damaging the coating and allowing water to seep in.

Adhesives remediate both of these issues. They reduce the vibration and they also don’t require you to damage the metal’s coating when installed. This is because adhesives don’t require you to drill holes into the metal.

So in summary, adhesives (whilst offering anti-corrosive properties) will only help reduce the risk. They certainly won’t eliminate the risk of corrosion. Whereas mechanical fasteners are much more likely to increase the threat.

Are Glue or Screws better? Article

Can you avoid filiform corrosion?

No, you cannot completely eliminate the risk of filiform corrosion. But as described above, there are plenty of methods that can reduce the threat it poses.

We understand that filiform corrosion and its long-term effects may concern you. However, we hope that this article has given you a bit more insight into the topic and provided some tips on how to reduce its risk.

As adhesive specialists, we are aware of the effects adhesives can have on reducing the threat posed by filiform corrosion. We also understand that you want to make sure your project isn’t going to go wrong. Filiform corrosion is just one risk you need to mitigate.

If you are in the transportation industry, you want to make sure your product isn’t going to fail. Failure can be fatal. 

Ultimately, as long as your metal surface is entirely well-covered by the coating of choice, the threat of filiform corrosion will be reduced. 

But if you want to ensure operator error isn’t going to affect when it comes to adhesives, we advise you to download our fundamental guide to adhesives. It will give you a guide to all things adhesives. You can download using the button below.

Become an adhesive expert and downoad our eBook

Thomas Besley

Thomas is the Content Manager here at Forgeway. Thomas' job is to translate the technical jargon from the ivory tower of academia into easy-to-read content that everyone can understand. Forgeway's mission is to answer every question our customers and prospective clients ask, or are apprehensive to ask.