Weld seam corrosion

P pitting, S weld seam corrosion, SCC stress corrosion cracking, () slight corrosion, n.a. no corrosive attack detectable... 

For instance, between the blank surface of a silicon-steadied shipbuilding steel and scale surfaces in synthetic seawater a potential difference of 400 mV is measured. The corrosion current that then flows between surfaces of the same size is equivalent to a corrosion rate of 0.350 mm/a (13.8 mpy). Considering the area ratios at a welded connection, this results in welding seam corrosion of up to 3 mm/a (118 mpy) [57]. 

Weldments subjected to corrosive attack over a period of time may crack adjacent to the weld seams if the residual stresses are not removed. Gas—tungsten arc welding and gas—metal arc welding ate recommended for joining magnesium, the former for thinner materials and the latter for thicker materials. Maintaining a protective atmosphere is a critical issue in welding these alloys. 

Two sections of steel condenser tubing experienced considerable metal loss from internal surfaces. An old section contained a perforation the newer section had not failed. A stratified oxide and deposit layer overlaid all internal surfaces (Fig. 5.14). Corrosion was severe along a longitudinal weld seam in the older section (Fig. 5.15). Differential oxygen concentration cells operated beneath the heavy accumulation of corrosion products and deposits. The older tube perforated along a weld seam. 

Attack at welds due to bacteria is possible, but it is not nearly so common as is often supposed. Because of residual stresses, microstruc-tural irregularities, compositional variation, and surface irregularities, welds show a predisposition to corrode preferentially by most corrosion mechanisms. Attack is common along incompletely closed weld seams such as at butt welds in light-gauge stainless steel tubing (Fig. 6.9A and B). Attack at carbon steel welds may occur. Figure 6.10 shows a severely corroded carbon steel pipe from a service water sys-... 

Figure 6.9 Irregular deposit and corrosion-product mounds containing concentrations of sulfate-reducing bacteria on the internal surface of a 316 stainless steel transfer line carrying a starch-clay mixture used to coat paper material. Attack only occurred along incompletely closed weld seams, with many perforations. Note the heat tint, partially obscured by the deposit mounds, along the circumferential weld.

Chemical removal of deposits and corrosion products revealed the appearance of the groove (Fig. 14.5). The crevice formed by the incompletely fused weld seam fostered the establishment of differential concentration cells (see Chap. 2). This resulted in localized corrosion and eventual perforation through the greatly thinned tube wall at the bottom of the crevice. The tubercle, which is composed of corrosion products, is a simple result of the corrosion process occurring locally within the crevice. 

Titanium stabilised fillers should not be used in argon-arc welding as titanium will be vaporised and its effectiveness as a stabiliser lost. Carburising the weld seam by pick-up from surface contamination, electrode coatings or the arc atmosphere leads to increased tendency to intercrystalline corrosion. 

Reviewing the calculation methods as implemented in the various pressure vessel codes (see Table 4.3-1) shows that these are based on the scientific formulae as per Table 4.3-2 but partly appear in a different shape due to approximations, simplifications, additional safety margins and other factors taking care of manufacturing tolerances, weakening by corrosion and welding seams [3] [4] [5] involved. 

Zinc—Cobalt. Alloys of Zn—Co usually contain 0.3—0.8% cobalt. Higher cobalt alloys, from 4—8%, have shown better salt spray resistance (156), but the commonly plated alloy is 0.3—0.8%. One automotive company specifies 0.3—1.0%. Cobalt is expensive, and economics favor the lower alloys. Costs have been quoted for zinc—cobalt at 1.2 times the cost of chloride zinc, with zinc—nickel alloys at 1.5—1.6 times the chloride zinc. Deposits can be very bright, but the improved corrosion resistance advantage requires yellow or bronze chromates. Alkaline baths give fewer problems in plating components with lapped, spot-welded seams. 

Corrosion Resistance of Nickel-based Alloys. Nickel-based alloys are solid solutions based on nickel. Nickel-based alloys used for low-temperature aqueous or condensed systems are generally known as corrosion-resistant alloys (CRA), and nickel alloys used for high-temperature applications are known as heat-resistant alloys (HRA), high-temperature alloys (HTA), or superalloys. The corrosion performance could change due to the presence of second phase or a weld seam. (Rebak)5... 

It has been shown that welds provide unique environments for the colonization of SRB with the subsequent production of sulfides that affect the weld seam surface of the heat-affected zone. Exposure of sulfide-derived surfaces to fresh, aerated seawater resulted in rapid spalling on the downstream side of weld seams. The bared surfaces became anodic to the sulfide-coated weld root, initiating and accelerating localized corrosion. (Dexter)5... 

For the final choice of material in the case of extreme operating conditions or corrosive or erosive media, plant components that are particularly at risk are constructed from the material in question and installed in the plant, monitored during operation of the test plant, and, when the test is finished, subjected to destructive testing. These are the most meaningful tests, since the conditions are closest to those of the industrial plant. Material samples that are to be tested for stress corrosion cracking are best installed in the miniplant under mechanical stress (material samples with welded seams). 

In stress corrosion cracking, the material breaks as the result of mechanical stress under the influence of a corrosive medium. Stress corrosion cracking is characterized by the presence of deep intergranular or intercrystalline cracks that are generally not externally evident. It can be caused by inherent stress, which can be due to cold working or arise near a welding seam. 

Corrosion Resistance of Nickel-Based Alloys There are three types of nickel-based alloys (i) CRA (ii) heat-resistant alloys and (hi) high-temperature alloys or super alloys. The corrosion performance can change because of the presence of a weld seam as a second phase. 

Many of the components of the vehicle such as fasteners, handles, brackets, and frame were made from 1010 carbon steel, which is prone to corrosion with ease. These parts corroded on almost every HMMWV in service leading to extensive repairs and maintenance. Vehicle parts such as the engine compartment, suspension, and steering, body, underbody, and other miscellaneous parts such as welded seams, fuel tank assemblies, nuts, bolts, fasteners, and frame suffered corrosion and the percentage of vehicles affected was in the range of 13-76%. 

In practice, the considerable influence of the stress level can also be seen by the fact that weld seams are preferentially attacked by stress corrosion cracking on account of shrinkage stresses and the stresses caused by structural changes due to the effect of heat input during welding. 

The method also allows the influence of the ferrite content in weld seams on stress corrosion cracking susceptibility to be determined. For a very low ferrite content the weld metal is significantly more susceptible to stress corrosion cracking, which is verified by the very low elongation values in Figure 17. 

The mainly similar behaviour of unalloyed and low-alloyed structural steels concerning the corrosion rate in seawater is finally due to the fact that the rusting process is controlled by oxygen access, which is not a material-dependent factor. On the other hand, the kinetics of the partial reaction iron dissolution, which determines the rest potential, is a material-dependent factor. The electrically conductive contact of materials or material parts with different rest potentials results in the formation of elements that may cause increased local corrosion depending on the potential level and area ratio. In practice, such elements occur at welding seams. 

Bending samples made of the steel S500 (UNS T11342,1.3247), for example, hardened to 467-481 HV30 by quenching in water, showed cracking in the heat affected zone of the welding seam. The cracks only occurred under exposure in the splash zone, in which the pH was low under mst bhsters due to hydrolysis of the corrosion products [65]. 

These tests provide a basis for conclusions concerning potential stress corrosion cracking under a completely static load. In plastic stress loads, especially in case of low elongation rates, the softer base material in comparison to the hardened welding seam zones may be at risk for hydrogen-induced stress corrosion cracking [25, 67-69]. 

Local corrosion is only to be expected on the state-of-the-art overalloyed welding seams in the presence of welding faults. 

Corrosion prognosis for the welding seam area) (in German)... 

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