Corrosion failures of welds can occur even when the proper base metal and filler metal have been selected, industry codes and standards have been followed, and welds have been deposited that possess full weld penetration and have proper shape and contour. This chapter describes some of the general characteristics associated with the corrosion of weldments. The role of macro- and microcompositional variations, a feature common to weldments, is emphasized in this chapter to bring out differences that need to be realized in comparing the corrosion of weldments to that of wrought materials. The discussion covers the factors influencing corrosion of weldments, microstructural features of weld microstructures, various forms of weld corrosion, and welding practice to minimize corrosion.

Carbon and low-alloy steels are the most frequently welded metallic materials, and much of the welding metallurgy research has focused on this class of materials. Key metallurgical factors of interest include an understanding of the solidification of welds, microstructure of the weld and heat-affected zone (HAZ), solid-state phase transformations during welding, control of toughness in the HAZ, the effects of preheating and postweld heat treatment, and weld discontinuities. This chapter provides information on the classification of steels and the welding characteristics of each class. It describes the issues related to corrosion of carbon steel weldments and remedial measures that have proven successful in specific cases. The major forms of environmentally assisted cracking affecting weldment corrosion are covered. The chapter concludes with a discussion of the effects of welding practice on weldment corrosion.

Austenitic stainless steels exhibit a single-phase, face-centered cubic structure that is maintained over a wide range of temperatures. This chapter provides a basic understanding of grade designations, properties, and welding considerations of austenitic stainless steels. It also discusses general types of corrosive attack and their effects on service integrity as well as detection and control measures. The five corrosive attack mechanisms covered are intergranular corrosion, preferential attack associated with weld metal precipitates, pitting and crevice corrosion, stress-corrosion cracking, and microbiologically influenced corrosion.

Ferritic stainless steels are essentially iron-chromium alloys with body-centered cubic crystal structures. Chromium content is usually in the range of 11 to 30%. The primary advantage of the ferritic stainless steels, and in particular the high-chromium, high-molybdenum grades, is their excellent stress-corrosion cracking resistance and good resistance to pitting and crevice corrosion in chloride environments. This chapter provides information on the classifications, properties, and general welding considerations of ferritic stainless steels. The emphasis is placed on intergranular corrosion, which is the most common cause of failure in ferritic stainless steel weldments. Two case histories involving intergranular corrosion failures of ferritic stainless steel weldments are included. A brief discussion on hydrogen embrittlement is also provided.

Duplex stainless steels are two-phase alloys based on the iron-chromium-nickel system. Duplex stainless steels offer corrosion resistance and cost advantages over the common austenitic stainless steels. Although there are some problems with welding duplex alloys, considerable progress has been made in defining the correct parameters and chemistry modifications for achieving sound welds. This chapter provides a basic understanding of the development, grade designations, microstructure, properties, and general welding considerations of duplex stainless steel. It also discusses the influence of ferrite-austenite balance on corrosion resistance and the influence of different welding conditions on various material properties of alloy 2205 (UNS S31803).

Martensitic stainless steels are essentially iron-chromium-carbon alloys that possess a body-centered tetragonal crystal structure (martensitic) in the hardened condition. Martensitic stainless steels are similar to plain carbon or low-alloy steels that are austenitized, hardened by quenching, and then tempered for increased ductility and toughness. This chapter provides a basic understanding of grade designations, properties, corrosion resistance, and general welding considerations of martensitic stainless steels. It also discusses the causes for hydrogen-induced cracking in martensitic stainless steels and describes sulfide stress corrosion resistance of type 410 weldments.

Nickel-base alloys used for low-temperature aqueous corrosion are commonly referred to as corrosion-resistant alloys (CRAs), and nickel alloys used for high-temperature applications are known as heat-resistant alloys, high-temperature alloys, or superalloys. The emphasis in this chapter is on the CRAs and in particular nickel-chromium-molybdenum alloys. The chapter provides a basic understanding of general welding considerations and describes the welding metallurgy of molybdenum-containing CRAs and of nickel-copper, nickel-chromium, and nickel-chromium-iron CRAs. It discusses the corrosion behavior of nickel-molybdenum alloys and nickel-chromium-molybdenum alloys. Information on the phase stability and corrosion behavior of nickel-base alloys is also included.

The nonferrous alloys described in this chapter include aluminum and aluminum alloys, copper and copper alloys, titanium and titanium alloys, zirconium and zirconium alloys, and tantalum and tantalum alloys. Some of the factors that affect the corrosion performance of welded nonferrous assemblies include galvanic effects, crevices, assembly stresses in products susceptible to stress-corrosion cracking, and hydrogen pickup and subsequent cracking. The emphasis is placed on the compositions, general welding considerations, and corrosion behavior of these alloys.

Many factors must be considered when welding dissimilar metals, and adequate procedures for the various metals and sizes of interest for a specific application must be developed and qualified. Most combinations of dissimilar metals can be joined by solid-state welding (diffusion welding, explosion welding, friction welding, or ultrasonic welding), brazing, or soldering where alloying between the metals is normally insignificant. This chapter describes the factors influencing joint integrity and discusses the corrosion behavior of dissimilar metal weldments.

This chapter reviews weld corrosion in three key application areas: petroleum refining and petrochemical operations, boiling water reactor piping systems, and components used in pulp and paper plants. The discussion of each area addresses general design and service characteristics, types of weld corrosion issues, and prevention or mitigation strategies.

This chapter addresses in-service monitoring and corrosion testing of weldments. Three categories of corrosion monitoring are discussed: direct testing of coupons, electrochemical techniques, and nondestructive testing techniques. The majority of the test methods for evaluating corrosion of weldments are used to assess intergranular corrosion of stainless steels and high-nickel alloys. Other applicable tests evaluate pitting and crevice corrosion, stress-corrosion cracking, and microbiologically influenced corrosion. Each of these test methods is reviewed in this chapter.

It is well established that solidification behavior in the fusion zone controls the size and shape of grains, the extent of segregation, and the distribution of inclusions and defects such as porosity and hot cracks. Since the properties and integrity of the weld metal depend on the solidification behavior and the resulting microstructural characteristics, understanding weld pool solidification behavior is essential. This article provides a general introduction of key welding variables including solidification of the weld metal or fusion zone and microstructure of the weld and heat-affected zone. It discusses the effects of welding on microstructure and the causes and remedies of common welding flaws.

The formation of defects in materials that have been fusion welded is a major concern in the design of welded assemblies. This article describes four types of defects that, in particular, have been the focus of much attention because of the magnitude of their impact on product quality. Colloquially, these four defect types are known as hot cracks, heat-affected zone microfissures, cold cracks, and lamellar tearing.

This article reviews the fundamental and specific factors that control the properties of steel weldments in both the weld metal and heat-affected zone (HAZ). The influence of welding processes, welding consumables, and welding parameters on the weldment properties is emphasized. The service properties of weldments in corrosive environments are considered and subjected to cyclic loading. The article summarizes the effects of major alloying elements in carbon and low-alloy steels on HAZ microstructure and toughness. It discusses the processes involved in controlling toughness in the HAZ and the selection of the proper filler metal. The article provides a comparison between single-pass and multipass welding and describes the effect of welding procedures on weldment properties and the effects of residual stresses on the service behavior of welded structures. It also describes the fatigue strength and fracture toughness of welded structures. The article reviews various types of corrosion of weldments.

Nickel-base alloys are generally used in harsh environments that demand either corrosion resistance or high-temperature strength. This article first describes the general welding characteristics of nickel-base alloys. It then describes the weldability of solid-solution nickel-base alloys in terms of grain boundary precipitation, grain growth, and hot cracking in the heat-affected zone; fusion zone segregation and porosity; and postweld heat treatments. Next, the article analyzes the welding characteristics of dissimilar and clad materials. This is followed by sections summarizing the various types and general weldability of age-hardened nickel-base alloys. The article then discusses the composition, welding metallurgy, and properties of cast nickel-base superalloys. Finally, it provides information on the welding of dissimilar metals, filler metal selection for welding clad materials and for overlay cladding, service conditions during repair, and welding procedural idiosyncrasies of cobalt-base alloys.