Rust,corrosion an its effect

Why Does Stainless Steel Rust After Welding and What You Can Do to Prevent Your Welds from Corroding

10

Apr

Stainless Steel is Highly Resistant to Corrosion but can Still Rust

The general perception towards stainless steel is that it is a super alloy that never rusts. Well, this assumption is anything but true. The reality is that stainless steel does rust.

The word “stain-less” does not imply free from stain or “stain-impossible”. It simply means that the alloy stains less.

This is exactly the case with stainless steel. When compared to other metals and alloys, stainless steel is the most resistant to corrosion.

Ordinary carbon or alloy steels are not in the same league as stainless steel when it comes to corrosion resistance.

The secret ingredient is chromium.

A common misconception among people is that stainless steel requires a huge amount of chromium. This is far from the truth. Only 12% of chromium is needed to make any steel stainless. Once this seemingly insignificant amount of chromium is added, the steel is able to resist rust and stain much less than other variants of the same metal.

So why does stainless steel rust at all?

Corrosion Chart

Corrosion possibility

Electric Conductivity

Chloride Con

Sulphides

ms/m

mg/L

mg/l

Risky

4.0 m

200

100

Caution

from 2 - 4

100 to 200

50 to 100

Safe

less than 2

Less than 100

less than 50

Aggressive Conditions Cause Rust on Stainless Steel 

Although stainless steel does corrode, it is important to note that the alloy will not rust under normal atmospheric or water based environments.

In other words, the corrosion of stainless steel takes place in specific aggressive conditions which are conducive for corrosion.

Basic types of stainless steel like 304 and 316 may corrode easier in certain environments and then a more highly alloyed stainless steel can be used.

Therefore, the use of basic forms of stainless steel is not recommended in more hostile conditions.

To make sure that the stainless steel application or structure remains unharmed, unscathed and intact in more disadvantageous conditions, the use of highly alloyed stainless steel is required.

Corrosion Resistance is Quantitative

In other words, stainless steels are more or less corrosion resistant and they work very well in almost every environment.

The characteristic of corrosion resistance is both qualitative as well as quantitative.

Each type of stainless steel has a certain degree of corrosion resistance. Once the limit of resistance is crossed, the formation of rust is bound to occur.

The limit of the stainless steel corrosion resistance is determined by what goes into making the stainless steel in the first place. This means that the limit of resistance is dependent on the constituent elements.

As a result of this, different grades of stainless steel respond differently when left in an environment that aids corrosion.

The grade of stainless steel is not the only way to judge its corrosion resistance.

The detailing and workmanship of each and every stainless steel goes a long way in deciding how vulnerable the steel will be to staining and corrosion.

6 Common Types and Forms of Stainless Steel Corrosion

Understanding the corrosion mechanisms of stainless steels helps us understand why stainless steels rust. Corrosion mechanisms in stainless steel can be broadly divided into 6 categories. They are:

a) Pitting Corrosion

This is a localized version of corrosion that takes place in stainless steel when it is exposed to environments that contain chlorides.

b) Crevice Corrosion

This is another form of localized corrosion that is triggered when the oxygen levels are extremely low in a crevice. This particular type of corrosion is not much of a problem unless the stainless steel is in a stagnant solution where chlorides can accumulate.

c) Bimetallic corrosion

When dissimilar metals in a common electrolyte come into contact with another, then bimetallic corrosion, also known as galvanic corrosion can take place. The most common scenario is stainless steel corroding in rain.

d) Stress Corrosion Cracking

Tensile stresses coupled with specific environmental conditions lead to stress corrosion cracking.

e) General Corrosion

When the stainless steel has a pH value of less than 1, then general corrosion takes place.

f) Intergranular Attack

When you heat austenitic stainless steel at around 450 to 850 degrees Celsius, the carbon in the steel converts to grain boundaries which lead to corrosion.

Removal and Prevention of Rust on Stainless Steel Welds

Welding is one of the most common types of metal fabrication.

Although useful, this process can seriously damage the anti-corrosive properties of stainless steel surfaces.

Melting, as a result of high temperatures during welding, deprives the steel of its protective layer and, therefore, its innate ability to resist corrosion.

For this reason, it is important to pay equal attention to two related but slightly different processes:

Removal of rust, cross-contamination, and heat tints – If rust has already appeared on your weld, it is necessary to remove it completely, but it is also important to get rid of all contaminating particles that occur as a result of industrial metal processing. In addition, you will need to remove the heat tints that appear around the weld bead and within the heat-affected zone.

Repassivation of the surface – Under normal conditions, the protective passive layer will self-restore if mildly damaged. However, this is usually not the case when discussing the side-effects of welding and other forms of metal fabrication. Therefore, your intervention is essential in preventing future corrosion.

But what exactly can you do in order to effectively clean your welds and achieve surface (re) passivation?

Luckily, you can now do both things simultaneously by utilizing the electrochemical weld cleaning method.