# Pitting Corrosion in AISI 304 Rolled Stainless Steel Welding at Different Deformation Levels
## 1 Introduction  
The objective of this study is to analyze the resistance of AISI 304 welds to pitting corrosion when they have been subjected to different levels of cold rolling, as well as to establish a correlation between the pitting corrosion and cold rolling deformation level. The pitting corrosion, initiated at certain points of the surface due to the non-uniformity of the passive layer, is a localized redox (reduction-oxidation) process that occurs inside small pits on the surface of passivated metals in aqueous media. This kind of reaction requires the balancing of both the reduction and oxidation half-reactions. The anodic reaction that takes place inside the pit leads to the dissolution of iron (Fe \(\rightarrow\) Fe\({}^{2+}\) + 2e\({}^{-}\)) while the cathodic reaction, that makes use of the electrons liberated by the anodic reaction, leads to the apparition of hydroxide (1/2O\({}_{2}\) + H\({}_{2}\)O + 2e\({}^{-}\) \(\rightarrow\) 2(OH\({}^{-}\))). As inside the pit, the electrolyte solution becomes positively charged due to the presence of Fe\({}^{2+}\) ions, the anionic species (Cl\({}^{-}\)) in the electrolyte are attracted there, increasing the acidity inside the pit and accelerating the corrosion process according to the reactions:  
\[ \begin{array}{ll}\text{FeCl}_{3}\rightarrow\text{Fe}^{3+}+3\text{Cl}^{-}& \text{(outside the pit)}\\ \text{Fe}^{3+}+3\text{(OH)}^{-}\rightarrow\text{Fe(OH)}_{3}&\text{(outside the pit)}\\ \text{Fe}^{2+}+2\text{Cl}^{-}\rightarrow\text{FeCl}_{2}&\text{(at the pit)}\\ \text{FeCl}_{2}+2\text{H}_{2}\text{O}\rightarrow\text{Fe(OH)}_{2}+2\text{HCl}& \text{(at the pit)}\\ \end{array} \]Furthermore, the solid Fe(OH)\({}_{3}\) deposited around the pit increases the separation of the inside of the pit from the rest of the electrolyte. The presence of chemical species that provide a high concentration of Cl\({}^{-}\) in the aqueous media is needed to accelerate the process of corrosion in the pit. AISI 304 stainless steel is widely used in the manufacturing of both industrial and domestic products, mainly due to the combination of characteristics that presents: Good mechanical properties (high strength, ductility and malleability); non-magnetic behavior; weldability; resistance to corrosion and low cost [1; 2; 3; 4]. It is used in applications that entail a high stress level and/or exposure to corrosive atmospheres, like in nuclear power plants [5; 6], marine environments [7; 8] or in large facilities [9]. Particularly, AISI 304 rolled stainless steel is one of the most frequently used types of stainless steel, whereby it is needful to be aware of the mechanisms that lead to its deterioration in order to foresee, restrict and/or quantify the economic costs that could be derived. Corrosion is one of the key factors to consider, as it can cause failures or malfunction in all types of products and facilities. Specially, when they get in contact with corrosive agents or when they are used in conditions or participate in processes that bring on corrosion [8], like welding and cold forming. The anti-corrosion behavior of welded stainless steels has been studied previously [10]. AISI 304 stainless steel can be welded by almost all the common welding techniques [11]. In arc welding the use of a low carbon content stainless steel as a filler rod reduces chromium carbide, which improves corrosion resistance [7; 12]. However, corrosion resistance can be affected when stainless steel is subjected to cold rolling [13; 14]. The friction and compression forces under conditions of dynamic plastic deformation cause the material fibers to stretch while its stress level rises and its hardness increases [15]. Beyond a certain level of deformation different types of defects and even failure can occur. If the process is under control these defects can be avoided, but the relieve of internal stresses requires an annealing treatment [16].