Corrosion of Welds – part 1

Corrosion of Welds

The importance of welding in various industries cannot be stated enough. Weld failure occurs due to two major reasons – improper welding technique, and corrosion of welds. The corrosion of welds has been a concern for almost all industries.

The major reasons for the corrosion can be broadly noted as below –

1. The difference in composition w.r.t the base metal
2. Microstructural variation w.r.t the base metal
3. Local heat zones around the weld
4. Changes in base metal microstructure due to the heat zones
5. Mechanical property variation due to any of the above mentioned reasons

The following corrosion conditions need to be considered here –

1. Medium
2. Temperature
3. Pressure
4. location

The type of failure may also depend on various factors such as –

1. base material – type, composition, metallurgical details
2. filler material – type, composition, metallurgical details
3. type of welding technique
4. Welding parameters

It is pertinent to consider all three categories of information to come to the most accurate resolution for any corrosion issue a weld may encounter.

The next step is to note in what form the corrosion may present itself.

Different forms of Corrosion in Welding

The typical forms of corrosion seen in welds are –

1. Uniform
2. Galvanic
3. Pitting
4. Intergranular – This is further sub-divided as weld decay and knife line attack (KLA)
5. Stress corrosion cracking – hydrogen-induced underbead cracking
6. Microbially influenced corrosion

In popular literature in the form of books and websites, the problem of corrosion in welds is discussed in the following two ways –

1. Description of all the filler/metal combinations and their respective corrosion types.
2. A list of all possible corrosion problems with no distinction between the forms of corrosion and the corrosion due to specific electrolytic medium of an application

This leads to confusion and the readers are unable to retain much of the information.

This series of articles aims to bring together the information in a simple and palatable way.

Its concise form will help the readers to relate the corrosion form to the weld immediately.

A short description of the basics of corrosion mechanisms is also included to initiate the reader into the field of corrosion theory.

Related Reading: What is Corrosion & Its Types.

CORROSION CELL

The formation of a corrosion cell is pertinent to the initiation and propagation of corrosion. A corrosion cell is composed of five components.

They are as below-

1. Anode – Area which dissolves, aka the corroding area.
2. Cathode – Area which supports cathodic reactions, aka does not corrode.
3. Electrolyte – Liquid medium which is the very reason for corrosion to occur.
4. Electrical connection – Metallic connection between anodic and cathodic areas.
5. Potential difference – difference in the tendency of the anode and cathode to either reject/or accept electrons.

If even one of the above components is eliminated, corrosion WILL NOT OCCUR.

In a weld region, all these components are present on the same surface, and they are microscopic to begin with.

UNIFORM CORROSION

Uniform corrosion is characterized by the appearance of corrosion products over the entire surface of the weld zone, and some of the neighbouring zones.

Its mechanism involves the formation of corrosion cells all over the weld. Corrosion cells are formed due to the differing grain structure.

As shown in the image below, a corrosion cell is formed on the weld which is essentially inhomogeneous.

The homogeneity may be due to surface discontinuities and differing grain structures in the same zone.

The striations in the image below show the differences in the adjoining grains. ‘A’ represents the anode and ‘C’ represents the cathode.

Typically, there are multiple anodes and cathode places in an alternate fashion. At the cathodic sites, the oxygen and water, and any other cathodic species come together. They take electrons from the metal and a reduction reaction occurs, as shown below –

O2 + 2H2O + 4 e- → 4OH-

This electron transfer implies that there is a metallic atom that has lost an electron and now has become a cation with the following reaction –

M → M + e-

As a result, the metal loss will be at the anodes and the deposition of the corrosion products will start at the cathode and may cover the entire surface.

This is called uniform corrosion as it may occur over the entire weld surface. So let us tabulate the major factors and combinations prone to this type of corrosion –

Mechanical factors	Metallurgical factors	Susceptible alloys
Pre-weld heat treatment of base metal	Segregation of alloying elements	Carbon steel
Weld temperature	Differing adjoining grains	Low alloy steel
Post weld brushing	The phase difference in a single weld zone	Stainless steel in an ‘oxide film-dissolving’ environment, i.e alkaline environment
	Diffusion of Mo and Tg from dendrite core to interdendritic region

GALVANIC CORROSION

Galvanic corrosion is the most common corrosion in welds. It occurs when the filler and base metals are dissimilar materials.

The electrode potentials of the base and the filler, i.e. their tendency to form ions is essentially different.

This can be traced from the Galvanic series shown below. The series gives a list of alloys with their electrode potentials.

Let us choose a combination for weld/base – mild steel/SS 410. From the galvanic series, the potential for mild steel is about -0.6 V vs SCE (saturated calomel electrode) and that for SS 410 is about -0.3 V vs SCE.

The mild steel is more electronegative than SS 410. Thus, when we use them together as weld/base combination, the more electronegative metal is more susceptible to corrosion. This implies that the mild steel weld will undergo corrosion.

This happens because here, the mild steel weld is now the anode and the SS 410 base is the cathode. The metal loss will occur from the anode.

Now, let us take another example. Suppose the weld/base combination is SS 321/ SS 316. In this case, look at the green arrows. The electrode potential for SS 321 is -0.1 V, and that for SS 316 is -0.05 V.

Thus, even if both are highly corrosion resistant SS alloys, their combination has made SS 321 the anode and SS 316 the cathode. So, if there is any moisture deposition, the more electronegative SS 321 will preferentially corrode, once its oxide film is damaged.

This is called galvanic corrosion. Further, even if the two alloys are not dissimilar, it may occur due to local variations in the compositions of the adjoining weld zone, HAZ and base metal.

. So let us tabulate the major factors and combinations prone to this type of corrosion –

Mechanical factors	Metallurgical factors	Susceptible alloys
Selection of vastly dissimilar weld/base combinations	Compositional variation in base, HAZ, weld	All combinations of dissimilar alloys
Oxide tin left after weld leading to localized cathode/anode formation	The large difference in the composition of filler and base alloy

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REFERENCES

1. https://www.asminternational.org/documents/10192/1849770/ACFAB86.pdf
2. Corrosion engineering, Mars G Fontan

Article Written by:

Dr. Shyama D Ranade (Corrospective.com)

Dr. Shyama Ranande is a highly skilled consultant and instructor with extensive experience in the field of corrosion, coatings, cathodic protection, and chemical treatment. With a focus on innovation and sustainability, she has also been involved in the development of novel bio-based smart coatings.