Is it a bad idea to just use stainless steel wherever rust is likely to occur? ~About galvanic corrosion~

Hello! This is Yamazato from Hanshin Neji Mexico. Our company is located in Queretaro, Mexico's central highlands, which is extremely dry. Even in a place like this (?!), we often get inquiries from customers asking how to deal with rust. Among these rust problems, I would like to write about galvanic corrosion today. I hope that this blog can be of some help to you in your manufacturing endeavors.

If I use stainless steel, will it rust more?! That's ridiculous!

Have you ever thought, "It's going to rust, so I'll use stainless steel!"? While that's not wrong, if you're not careful with the material you're using it against, it could rust faster and cause serious problems.

For example, imagine you're fastening various parts to a steel mold with iron bolts. In a humid environment, when you try to replace the bolts, they can become so rusted that they become impossible to remove. In such a situation, you might think, "Wouldn't it be okay to just change the bolts to stainless steel?" However, doing so could cause the mold to rust much faster than before, resulting in greater loss than the loss of the bolts themselves.

So why does rust occur when dissimilar metals like iron and stainless steel come into contact? To explain this, we first need to understand the deeper meaning of rust, but in this article I'd like to explain what happens when dissimilar metals come into contact.

What is galvanic corrosion?

Each metal has a property called "ionization tendency," the strength of which varies depending on the metal. The word "ionization" itself can be difficult to understand, so here I hope you understand that there are some differences in the properties of metals.

In an area where there is moisture (humidity, etc.), the greater the difference, the faster electricity will flow through the moisture. When electricity flows, metals with a stronger ionization tendency will corrode rapidly. So, why does corrosion occur when electricity flows? That raises the question, but I would like to explain this in more detail in my next blog.

(Ionization tendency table)

Stainless steel is an alloy steel, so it is not included in this table, but it is said to be roughly the same as copper. When comparing iron and stainless steel, they are quite different, so the difference in ionization tendency is large and they are a combination that allows electricity to flow easily. If the two come into contact with something that conducts electricity, such as water, the iron, which has a stronger ionization tendency, will corrode.

This is roughly called galvanic corrosion.

How to prevent galvanic corrosion?

As mentioned above, corrosion is caused by the difference in ionization tendencies of dissimilar metals, so using the same metals eliminates this problem. Therefore, the standard combination is to use iron nuts with iron bolts and stainless steel nuts with stainless steel bolts.

However, there are often designs where copper piping must be connected to iron pipes, or stainless steel bolts must be used with aluminum frames. When using these types of connections, countermeasures are necessary.

1. Insulation
Since corrosion occurs when electricity flows, insulation is achieved by using resin or rubber gaskets, washers, or sleeves to prevent direct contact between dissimilar metals.

2. Choose materials and surface treatments with as similar ionization tendencies as possible.
For example, using aluminum bolts with an aluminum frame eliminates the problem, as they are the same metal. However, due to cost and strength issues, there are times when stainless steel bolts, which have different ionization tendencies, must be used. In such cases, applying Geomet®, which has an ionization tendency similar to that of aluminum, is effective in preventing corrosion. As such, it is important to select materials and surface treatments based on the ionization tendencies of the metals being used.

Conclusion
So, what did you think? Did you get a general understanding of galvanic corrosion?

(1) Metals have different "ionization tendencies."

(2) When dissimilar metals with large differences in their ionization tendencies are in contact, electricity flows through water, etc., accelerating corrosion.
(3) The solution is to prevent electricity from passing through them and to select materials and surface treatments with the same or different ionization tendencies.

In the next blog, I'd like to delve deeper into the concept of rust. Knowing this will help you better understand galvanic corrosion and make it easier to imagine countermeasures!

If you have any corrosion issues, please feel free to contact Hanshin Neji. We'll do our best to find a solution!

(Editor's Note)
This is Yamazato from Hanshin Neji Mexico. I talk about something so personal, but I recently started playing tennis. It's a team run by the Japanese Association in Queretaro. A friend of mine plays, and my son wanted to join, so we started going at the end of last year. At first, I just send my son and play a little bit, but it's actually quite fun. I just can't seem to get any better (lol). I played soft tennis in middle school, so I can hit the ball with the racket, but when I do, it's a home run (a huge miss), and when I try to hit a low ball, it ends up hitting the net... It's a real struggle. But when I try it after being taught, it's nice when it works out somehow, and I'm hoping to get good enough that I won't be a nuisance to everyone!