Corrosion in welded joints

We present an in-depth analysis on the control of failures in the design of structures.

by Abel De La Cruz*

Coatings applied to structures exposed to the atmosphere often experience premature failures due to lack of caution in structural design or metalworking factors, from the point of view of corrosion protection.

It is very common that the person responsible for the design or supervision during metalworking construction does not repair in the most critical and vulnerable areas during the life in service of the structure, so if the corrosion protection begins in the engineering of the project, the responsibility for the quality of the metalworking will be in the hands of the builder.

The deterioration of the paint continues with the corrosion of the base metal, so the mechanical stability of a structure with manufacturing problems in terms of the risk of corrosive attack can be seriously compromised.

That is why, when it comes to metal mechanical constructions we must take into account the basic criteria on the design of steel structures that are going to be covered with paint systems and in this way avoid the premature appearance of corrosion problems and the degradation of the coating and the structure, this is how the ISO 12944-3 Standard "Design Considerations" gives us information on the different types of appropriate designs also of non-recommended practices, also indicating how they can be avoided before application and maintenance with paints. The same standard contemplates design considerations that facilitate the handling and transport of steel structures.

Another source of information with recommendations and technical requirements of structural design for the construction of tanks subject to immersion with high risk of corrosion attack, is provided by the NACE RP 0178-95 Standard: "Fabrication Details, Surface Finish Requirements and Proper Design Considerations for Tank and Vessels to Be Lined for Immersion Service" where reference is made precisely to vulnerable areas such as metal part joints, welding techniques, precautions in the quality of welding beads, etc.

Among the areas with the highest possibility of corrosive attack due to the manufacturing characteristics of a structure, tank, equipment, etc. are:
- Areas of the weld bead in general.
- Welding splashes, spot welding discontinuities and imperfections, interstices, cavities that allow the retention of moisture and contaminants.
- Areas of difficult access for painting.
- Bimetallic junctions or galvanic pairs.
- Water retention areas, humidity, contaminants in general.
- Areas with a bad design in the union of metal parts.
- Sharp edges or edges, etc.
- The aforementioned corrosion problems in welding will be dealt with in this article.

Therefore, it is common to see throughout the life of the installations painted in immersion or non-immersion, that the areas of special maintenance due to premature deterioration are the welded areas or the welding beads, and in a failure analysis, these areas are commonly the origin of the corrosion of the base metal.

Some of the factors that directly or indirectly aggravate the corrosion of welds are:
Possible difference in composition and electrochemical potential between the metal that constitutes the weld and the base metal (galvanic corrosion).

Metallographic structure of the weld and the metal neighboring it.
The presence of discontinuities and imperfections of the weld bead that allows the ease of accumulation of water or moisture, dust particles and other contaminants.
Peculiar geometry of the welded joint.
Presence of mechanical stresses.
 

Galvanic  Corrosion
In fusion welding, a series of structural differences are generated, which extend from the solidified tank to the base metal through a thermally affected area (Fig. I - see in printed or digital edition). It is therefore important that the chemical composition of the input product is very close to that of the base material, in which case it is not normal for significant galvanic or bimetallic corrosion problems to occur. However, these can manifest themselves in the midst of high aggressiveness, due to a moderate difference in electrochemical potential resulting from local changes in structures, stresses introduced during the bonding process or the fact that certain alloying elements have been "burned" in part.

In soft or brazing, there are usually considerable differences in electrochemical potential between the base material and the filler material. The situation poses serious problems when the weld metal, of marked difference in compassion from the base material, is at the same time more active (anodic behavior in a corrosion pile). In such a case, it is likely that the high cathodic area/nordic area ratio motivates a strong welding attack, capable of producing catastrophic failure of the structure (Fig. 2 - see in print or digital edition).

In order to avoid such a danger, an attempt is made to choose a contribution material that is more noble (cathodic) than the base metal. Soft solder, with filler alloys generally based on the tin-lead system, acts cathodically against ferrous materials, so it does not constitute a greater risk of galvanic corrosion. In the case of brazing of carbon and low-alloy steels, the filler materials are invariably noble compared to base steel (small cathode versus large anode), and galvanic corrosion problems are unlikely. Remember that galvanic corrosion is aggravated or becomes intense when there is a large cathode in front of a small anode.

Corrosion, concentration and aeration
Concentration differences in the electrolytic solution in contact with the metal surface cause the operation of concentration or differential aeration batteries if the dissolved substance is oxygen from the air. Such differences are usually motivated by the presence of cracks, by porous deposits or poorly adhered to the metal, deep inlets (roughness and folds), etc. to which oxygen has difficult access (by diffusion), or in which there is a depletion or accumulation of certain products or ions.

In welds this type of corrosion causes a deficient filling, incomplete penetration into the root of the bead, various irregularities, roughness of the weld bead, undercuts, porosities, cracks, porous weld splashes and poorly adhered on the metal etc. areas conducive to the penetration and condensation of moisture, for the operation of corrosion piles in metals exposed to the atmosphere. The effects of these batteries can be aggressive under conditions of permanent wetting of the welded joint.

The cracks under the projections or splashes of metal and slag residues from the welding operation are foci for the operation of the difference aeration piles. In this regard, it is recommended at the metal manufacturing stage, before the surface preparation operation, to remove or completely remove all these products to avoid, as far as possible, corrosion problems in the welded area.

Although not strictly linked to the weld itself, a number of problems of corrosion by differential aeration can occur as a result of an unsatisfactory design of the welded coupling, which should take into account the fundamental principle of avoiding loopholes (folds, grooves corners) and, in short, any place where it is difficult to renew the medium and where oxygen reaches with difficulty. Good drainage and ventilation of the bonded area is essential, ensuring the absence of electrolyte, and offering good accessibility for the subsequent application of the protective coating and maintenance inspections.

It must then be ensured that the welding beads are sufficiently smooth, without protrusions and cracks, roughness, holes, craters, fractures, roughness, splashes, which are difficult to cover efficiently by a paint system, so it must be the task of the quality control inspector of metalworking manufactures, that in the joints between structural parts the welding is continuous and free of defects for an adequate corrosion protection with Coatings.

As for the Specifications of Anticorrosive Protection Painting we must ensure the application of a layer of reinforcement paint (stripe coat) in these areas, before the application of the final layer of the painting system, ensuring a protective barrier layer. The "stripe coat" today is an indispensable application in any Painting Plan or Procedure and must be carried out through the use of brushes that ensure the penetration of the coating in the areas vulnerable to premature oxidation.

The Standard Guide for the Application of Paints SSPC - Guide 11: "Protecting Edges, Crevices and Irregular Steel Surfaces for Stripe Coating" provides us with the information and recommendations for the corrosion protection of areas vulnerable to corrosion due to the effects of mechanical metal design and fabrication and which we will discuss in a future publication.

Usually in a bilayer paint system is applied after the anticorrosive base layer and before the general finishing layer, a layer of finishing paint (additional) specifically in the hard-to-reach areas, welding beads, edges, interior angles, etc. It is recommended to extend the application up to 2 or 3 cms. of the area to be covered. This layer must have a different color to the finish and the base, so that it can be differentiated by the quality inspector and the application operators themselves.

Stress corrosion in welded joints
The expansions, thermal contractions and phase changes as a result of the welding process leave the metal with a system of residual stresses that, in the vicinity of the weld bead, are tensile stresses and their magnitude can reach the elastic limit and cause up to some permanent deformation. While favoring the condensation of moisture, the cavities and irregularities of the rough surface of the solidified metal in the weld act as stress concentrating zones.

It should not be surprising, therefore, that welded joints are exposed to stress corrosion cracking in case of contact with any of the specific means for the development of the phenomenon. They stimulate cracking as well as external stresses as well as internal tensions. To counteract the phenomenon, techniques must be selected, since defects such as non-metallic inclusions, selective precipitation of phases, gas pockets, undermining, fissures, etc. provide places for the concentration of tensile stresses and predispose cracking.

Precautions should be taken at the design stage of the project to avoid stress corrosion. However, a quantitative design against this form of corrosion is not always possible. The general recommendations are based on the fact that, for the phenomenon to occur, the conjunction of three factors is needed: a susceptible material, a stress of tension and an aggressive medium. Logically, the project must ensure that dangerous concentrations of the chemical species responsible for cracking (hydroxyl ions, chloride, nitrates, etc.) are not reached, through a careful control of the service conditions, avoiding configurations that lead to a local increase in the concentration of hazardous substance.  

* Eng. Mg. Abel De la Cruz Pérez. General Manager, Senior Consultant and Senior Facilitator American Consult Peru [email protected] - [email protected] Visit www.infocorrosion.com the first channel specialized in Surface Treatment, Corrosion Control Management and Asset Integrity in Latin America.