Reduced Corrosion Resistance Research Articles

Corrosion resistance of aluminum alloy AA7022 wire fabricated by friction stir extrusion

Abstract Friction stir extrusion (FSE) is known as an innovative manufacturing technology that makes it possible to directly produce wire via consolidation and extrusion of metal chips or solid billets. In this study, wire samples were produced using aluminum alloy AA7022 machining chips by the use of the FSE. To this end, the microstructures and mechanical properties of the base material (BM) and the extruded samples were investigated. The corrosion resistance of the given samples was also determined using potentiodynamic polarization technique. The results showed that the samples manufactured at higher rotational speeds possessed good surface quality, the process temperature and the grain size similarly increased following the rise in rotational speed, and the mechanical properties consequently decreased. Using the FSE led to crystallite refinement, increase in volume fraction of grain boundaries, as well as re-distribution of precipitates affecting corrosion resistance. Furthermore, the findings of the corrosion tests revealed that the produced samples by the FSE had adequate corrosion resistance and the growth in die rotation rate augmented current density and subsequently reduced corrosion resistance.

Effect of sulfate reducing Citrobacter sp. strain on the corrosion behavior of API X70 microalloyed pipeline steel

In this study, the effect of sulfate reducing Citrobacter sp. strain on the corrosion behavior of API X70 micro-alloyed pipeline steel has been evaluated in a simulated soil solution at different immersion times (varying from 7 days to 40 days) under both abiotic and biotic conditions using open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). The corrosion products and biofilm morphology developed on the metal surface at various immersion times were evaluated by field emission scanning electron microscopy (FESEM). Formation of a protective layer of corrosion products including ferric oxide, sulfide and carbon-based compounds accumulated on the underlying steel leads to an increase of the corrosion resistance for the first 21 days under abiotic condition. The results of electrochemical impedance spectroscopy in the solution containing bacteria show a significant reduction in charge transfer resistance from 400 Ω cm2 after day 7–55 Ω cm2 after day 21 due to the biofilm formation. This reduction in corrosion resistance is consistent with the high concentrations of sulfide produced by the metabolism of bacteria and the formation of a relatively compact and thick biofilm that is heterogeneously dispersed over the surface. In addition, a higher susceptibility to pitting corrosion is observed for samples immersed in the bacteria containing medium.

Quasi-static crack propagation in Ti-6Al-4V in inert and aggressive media

Abstract High strength-to-weight ratio Ti-6Al-4V alloy is used in many engineering applications. Its surface oxide film can protect the substrate from interaction with a lot of corrosive environments. Unfortunately, this surface layer can be damaged under mechanical and chemical actions with a consequent reduction in corrosion resistance. A characterization of the untreated alloy under quasi-static loading is here provided. Inert and aggressive environments have been investigated and the influence of the notch geometry of the alloy has also been analyzed.

Super Slipery Coating By ZnO Based on Aluminum and Its Anti-Corrosion Performance

Similar to the “lotus leaf effect”, super-hydrophobic surfaces have excellent self-cleaning characteristics and corrosion resistance. However, in the process of droplet condensation on a rough surface, the droplets on the surface can transform from the Wenzel-Cassie state to the Wenzel state, leading to failure of the superhydrophobic effect and greatly reduced corrosion resistance of the alloy surface. Aizenberg et al. first proposed the concept of a slippery liquid-infused porous surface (SLIPS), where lubricating oil is poured into the porous surface for preparation of super-slippery surfaces; a dynamic oil film forms on the surface, solving the problem of super-hydrophobic failure. Since then, research has been focused on mimicking nature and trying to fabricate such surfaces artificially. In this paper, double-layer zinc oxide was synthesized on an aluminum substrate by a sol-gel and hydrothermal method. The aluminum sheet coated with zinc oxide was infiltrated into the Dupon Krytox GPL104 lubricating oil for 2h to obtain an aluminum-based double-layer super-slippery surface. The surface has all the properties of a SLIPS, repelling all simple and complex liquids (including water, milk, coffee, juice, ink, and others); it also exhibits good anti-corrosion and antifouling properties. The corrosion resistance of the surface was tested by electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl aqueous solution (simulated seawater test); the result was four orders of magnitude higher than that of an untreated aluminum sheet. It shows good corrosion resistance, and the corrosion resistance did not change significantly after three months. This provides potential application prospects for aluminum alloy corrosion protection.

Influence of cerium content on the corrosion behavior of Al-Co-Ce amorphous alloys in 0.6 M NaCl solution

The effect of cerium content on the corrosion behavior of Al-Co-Ce amorphous alloys in 0.6 M NaCl solution was investigated by cyclic polarization, Mott-Schottky and X-ray photoelectron spectroscopy techniques. Results indicated that the open circuit potential of Al-Co-Ce amorphous alloys displayed a decreased tendency with the increase in Ce content, and the amorphous alloy with 4 at.% Ce presented both the lowest passive current density and donor density indicating the best corrosion resistance, while adding excess Ce led to the reduced corrosion resistance of Al-Co-Ce alloys. Furthermore, it was found that a low Ce content is beneficial to the formation of a more protective passive film on Al-Co-Ce amorphous alloys, and the corrosion inhibition reactions of Al-Co-Ce alloys in 0.6 M NaCl solution were changed with the increase in Ce content and the detailed reasons were discussed.

Crevice corrosion and environmentally assisted cracking of high-strength duplex stainless steels in simulated concrete pore solutions

This paper presents a study of crevice corrosion and environmentally assisted cracking (EAC) mechanisms in UNS S32205 and S32304 which were cold drawn to tensile strengths of approximately 1300 MPa. The study utilized a combination of electrochemical methods and slow strain rate testing to evaluate EAC susceptibility. UNS S32205 was not susceptible to crevice corrosion in stranded geometries at Cl− concentrations up to 1.0 M in alkaline and carbonated simulated concrete pore solutions. UNS S32304 did exhibit a reduction in corrosion resistance when tested in a stranded geometry. UNS S32205 and S32304 were not susceptible to stress corrosion cracking at Cl− concentrations up to 0.5 M in alkaline and carbonated solutions but were susceptible to hydrogen embrittlement with cathodic overprotection.

Ultrasonic Vibration Turning to Increase the Deposition Efficiency of a silica-based Sol-Gel Coating

Abstract Magnesium alloys are attracting more and more attention for producing temporary prosthetic devices thanks to their bioresorbable characteristics in human environment. However, they present a reduced corrosion resistance to body fluids, which still limits their applications to a great extent. One possible way to increase the corrosion performances is to coat the device with a suitable coating that provides a barrier to the body fluids corrosion. In this work, Ultrasonic Vibration Turning (UVT) is used to create a surface texture to the AZ31 magnesium alloy with the aim of improving the surface wettability and therefore helping the subsequent coating deposition. The obtained results showed that the surface texture induced by UVT contributed to increase the surface wettability of approximately 17% compared to the conventional turning case, regardless of the adopted cutting parameters. The UVT texture proved to improve the efficiency of the coating deposition since the thickness of the deposited sol-gel coating was increased when applied to UVT surfaces, especially at the lowest depth of cut and highest cutting speed that contributed to generate deeper dimples.

Graphene Reinforced Composites as Protective Coatings for Oil and Gas Pipelines.

Corrosion and corrosion-induced damage have resulted mostly in malfunctions and sometimes even in failures of metallic structures, including oil and gas pipelines. In this study, new high-performance composite coatings were developed by incorporating nanoparticles in the polymer resins with applications to oil and gas pipelines. The graphene nanoplatelets under different concentrations were used to prepare the epoxy-based nanocomposites and were then evaluated through mechanical and electrical tests. The integration of high-speed disk and ultrasonication were adopted as the dispersion technique to overcome nanoparticle agglomeration. Electron microscopy techniques were used to investigate the agglomeration. The new composites were qualitatively and quantitatively evaluated in terms of contact angle, surface roughness, adhesion to the substrate, corrosion resistance, and abrasion resistance. The results suggested that the composite with 0.5~1.0 wt.% of the graphene nanofillers led to the largest improvement in both mechanical and electrochemical properties. Distribution of nanoparticles in the matrix was observed using scanning electron microscopy and surface roughness using atomic force microscopy. Large agglomeration that was observed at the higher concentrations mainly resulted in the reduction of corrosion resistance and abrasion resistance.

Corrosion studies of Li, Na and Si doped Zn-Al alloy immersed in NaCl solutions

This work describes results concerning the corrosion behaviour of Li, Na and Si doped eutectic Zn-Al alloys. Corrosion properties were investigated with the use of several complementary electrochemical techniques: time resolved OCP analysis, potentiodynamic studies, rotating disc electrode (RDE) and electrochemical impedance spectroscopy (EIS). The corrosion potential (Ecorr) and corrosion current (icorr) were obtained using potentiodynamic tests. The corrosion studies revealed that the alloying addition to eutectic Zn-Al resulted in slightly lower corrosion resistance than observed for Zn-Al. The results indicate that the addition of Si reduced corrosion resistance the least, compared to Li and Na.

A study on the effect of compositing silver oxide nanoparticles by carbon on the ‎electrochemical behavior and electronic properties of zinc‐silver oxide batteries ‎

AbstractIn this study, the effect of compositing silver oxide nanoparticles by carbon on the electrochemical behavior and electronic properties of zinc‐silver oxide batteries have been investigated. For this purpose, firstly four silver oxide electrodes containing 5, 10, 15, and 20 wt% carbon powder were produced by powder metallurgy method. For the next step, all four silver oxide electrodes were sintered at 500°C for 10 minutes. Afterward and in order to investigate the microstructure, phase and elemental analysis of the electrodes were carried out using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X‐ray Diffraction (XRD), and Energy Dispersive Spectroscopy (EDS), respectively. Moreover, in order to investigate the effect of compositing silver oxide nanoparticles by carbon on the electrochemical behavior and electronic properties of zinc‐silver oxide, electrochemical tests (potentiodynamic polarization and electrochemical impedance spectroscopy) and electric discharge test in 1.4 wt%KOH electrolyte were carried out respectively. The microstructural observations revealed that increasing carbon content in the silver oxide electrodes results in increasing the apparent porosities in these electrodes. Investigating the phase and elemental analysis results showed that by increasing the content of carbon in the silver oxide electrode, the amount of Ag2O and AgO phases in this electrode reduces and also the extent of pure silver formation increases. Investigations on the results of electrochemical tests showed that increasing carbon content results in the reduction of corrosion resistance in silver oxide electrodes. Moreover, the results of electric discharge test revealed that the silver oxide electrode containing 10wt% carbon yields the highest energy efficiency in the zinc‐silver oxide batteries.

The influence of pH, rotating speed and Cu content reinforcement nano-particles on wear/corrosion response of Ni-P-Cu nano-composite coatings

Abstract Nano-composite coatings with nano particles contents from 1 to 7 gr/lit, with the electroless bath pHs of 4.5, 7, 9; and electroless bath rotating speeds of 200, 400 and 600 rpm were produced. Microstructural/elemental analyses were performed by various techniques. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques were used to study corrosion behavior. Wear resistance of the coatings was assessed by ball on disk wear test. Results showed that wear resistance of the coatings was improved at higher concentration of particles and by increasing the pH of the electroless bath as well. These conditions reduced corrosion resistance of the coatings, increased phases such as Cu2P7, CuP2, and reduced the P content in the Cu nano-composite coatings.

Improved Corrosion Resistance on Selective Laser Melting Produced Ti-5Cu Alloy after Heat Treatment.

A Ti-5Cu alloy produced by selective laser melting exhibits a nonuniform Ti2Cu phase structure, which contains a small amount of α' phase in melt pool boundaries thereby resulting in reduced corrosion resistance. The heat-treatment process proposed in this work eliminates the deleterious effect of α' phase and the Ti2Cu phase is refined using different cooling rates, which improves the corrosion resistance. The electrochemical results indicate that the heat-treated Ti-5Cu samples have similar corrosion behavior to pure CP-Ti. A slower cooling rate produces a larger spacing between the Ti2Cu phases in the microstructure of the sample, resulting in higher corrosion resistance. The corrosion behavior of SLM-produced Ti-5Cu and heat-treated counterparts with different microstructure are detailed discussed.

Coercivity enhancement of waste Nd-Fe-B magnets by Pr70Cu30 grain boundary diffusion process

Considerable quantities of Nd-Fe-B magnet wastes are produced every year worldwide. Some Nd-Fe-B magnet wastes in the bulk form, produced during manufacturing, have low coercivity and cannot meet the requirements for applications. Finding an effective way to reuse those wastes by improving the coercivity, without powdering or reproducing process, becomes very important for saving energy and raw materials in manufacture. In this work, the grain boundary diffusion process is carried out on waste Nd-Fe-B sintered magnets by using Pr70Cu30 as a diffusion medium. The effects of diffusion temperature, diffusion time, and annealing time on the magnetic properties of the magnets are investigated. It is found that the coercivity increases when the diffusion temperature increases from 500 to 800℃, the diffusion time increases from 1 to 3 h, or the annealing time increases from 1 to 3 h. By comparing the diffused sample with the simply heat treated sample, we find that the coercivity enhancement by grain boundary diffusion process indeed results from the infiltration of Pr and Cu elements. The coercivity of the magnet increases by 51.9%, from 7.88 kOe (1 Oe=79.5775 A/m) to 11.97 kOe, after 4-hour diffusion at 800℃ followed by 3-hour annealing, with a negligible reduction of remanence Br, achieving a 99.8% recovery of coercivity compared with the commercial N35 magnet. It is noted that 500℃ annealing for 3 h after 800℃ diffusion only slightly increases the coercivity by 4.6%, from 11.44 kOe to 11.97 kOe, which indicates that the annealing process after Pr-Cu grain boundary diffusion may be not indispensable. Based on the microstructure analysis, the diffusion of Pr and Cu is confirmed. However, the distributions of Pr and Cu are inhomogeneous within a range of tens of microns near the surface even though the diffusion has spread throughout the magnet. The structure of main phase grains separated by the continuous grain boundary phase is formed after the grain boundary diffusion process while the core-shell structure is not observed, which suggests that the modification of the grain boundary structure is the main reason for the coercivity improvement. Cu element plays an important role in forming continuous grain boundary phase. In addition, the electrochemical corrosion test shows that higher corrosion current is obtained in the diffused magnet than in the original magnet, though the corrosion potential is improved. The reduced corrosion resistance may be related to the increased RE-rich phase content and the formation of continuous grain boundary phase. The present work is of great importance for increasing the production yield of Nd-Fe-B magnets.

Large Strain Extrusion Machining under Cryogenic Cooling to Enhance Corrosion Resistance of Magnesium Alloys for Biomedical Applications

Magnesium alloys are attracting more and more attention for producing temporary prosthetic devices thanks to their bioresorbable characteristics in human environment. However, they present a reduced corrosion resistance to body fluids, which still limits their applications to a great extent. In this work, severe plastic deformation through large strain extrusion machining is explored, as a route for the surface and sub-surface modification of the AZ61 magnesium alloy with the aim of improving its corrosion resistance. Different process parameters, namely cooling condition and cutting speed, were adopted and their effects on the machined surface integrity and corrosion resistance were investigated. The obtained results show that cryogenic cooling applied to large strain extrusion machining can represent an efficient strategy to enhance the magnesium alloys poor corrosion behavior, regardless the adopted cutting speed.

About weldability and welding of Al alloys: case study and problem solving

Purpose: Among many disciplines within engineering, welding is probably one of the most inexact – rather more of an art than a science. As weldment is meant the complete joint comprising the weld metal, heat affected zones (HAZ) and the adjacent parent metal and should have the same properties as the parent metal. This paper aims provides a basic understanding of the metallurgical principles involved in how aluminium alloys achieve their strength and how welding can affect these properties. The most important and applied welding processes to Al alloys are here shortly introduced, as well as the preparation of parent metals prior to welding and good welding practice to avoid and/or keep under control defects and failures. Some case studies with possible failures will be introduced together with actions and suggestions to solve the observed problems. Design/methodology/approach: Two sheets of the EN AW 5454 (AlMg3Mn) alloy were weld with resistance welding process and after a mechanical processing of lamination was observed the presence of the defect. The microstructure of the defect as well as the welded part were evaluated with stereomicroscope (LEICA MS5), optical microscope (LEICA MEF4M), and with SEM analysis (LEO 1540 VP equipped with an energy dispersive X-ray spectroscope Oxford Link Pentafet). Findings: The well welded part was analysed with optical microscopy and electronic microscopy resulting with the attended mechanical properties. Micro-hardness indentations on the joint demonstrated the good mechanical properties of the joint while with the microscopic observations were identified the orientation and presence of precipitates typical of this alloy. In the defect, microscopically observations showed the presence of oxide inclusions. Research limitations/implications: There are a number of problems associated with the welding of aluminium and its alloys that make it difficult to achieve this ideal. The features and defects that may contribute to the loss of properties comprise the following: gas porosity, oxide inclusions and oxide filming, solidification (hot) cracking or hot tearing, reduced strength in the weld and HAZ, lack of fusion, reduced corrosion resistance and reduced electrical resistance. Originality/value: This case study illustrated clearly the importance of the cleaning on the surfaces to obtain a well welded joint ensuring the desired mechanical properties.

Investigation of the physical and mechanical properties of Ni–P and Ni–P–PTFE nanocomposite coatings deposited on aluminum alloy 7023

In this study, various properties of Ni–P and Ni–P–PTFE coating fabricated by electroless process were investigated. These coatings were applied on aircraft-grade aluminum samples. The results showed that the addition of nano-PTFE particles decreases coating rate from 7.1 µm/h to 6.1 µm/h and hardness from 510 HV to 200 HV. Also by increasing the pH value, coating rate increases from 1 µm/h to 7 µm/h. Increasing the solution temperature from 75 ℃ to 90 ℃ also increases the hardness of coating from 125 HV to 210 HV. The results showed that the heat treatment at 300 ℃ for 4 h increases the hardness up to 375 HV due to formation of Ni3P hard phase in Ni–P–PTFE coating. Addition of PTFE particles have improved tribological properties due to its lubricating effects and simultaneously, have reduced corrosion resistance compared to Ni–P coatings, so that the corrosion current for Al, Ni–P, and Ni–P–PTFE coatings is −880, −550, and −770 μA/cm2, respectively.

Synergistic effects of spray-coated hybrid carbon nanoparticles for enhanced electrical and thermal surface conductivity of CFRP laminates

Carbon fibre reinforced plastics (CFRPs) are intensively used in modern aircraft structures because of their superb specific mechanical properties. Unfortunately their electrical and thermal conductivities are not sufficiently high for some applications like electromagnetic interference (EMI) shielding and lighting strike protection (LSP). The addition of external metallic structures, such as aluminium or copper mesh, is generally required with a compromise in terms of increased mass and manufacturing cost as well as reduced corrosion resistance. In the present work spray coating of carbon nanoparticles was utilized as a simple method to locally increase the electrical and thermal suface conductivity of CFRPs. The combined use of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) synergistically reduced the CFRPs surface resistivity by four orders of magnitude (from 2–3 Ω/sq to 3 × 10−4 Ω/sq) and increased the thermal conductivity by more than 7 times (from 200 W m−1 K−1 to 1500 W m−1 K−1), opening up possibilities for the replacement of metallic mesh structures for EMI shielding and LSP. An analytical model was introduced based on a one-dimensional heat conduction approach to predict the effective thermal conductivity for the hybrid nanofiller coating layer and its findings showed good agreement with experimental data.

Electrical characteristics and discharge properties of hybrid plasma electrolytic oxidation on titanium

In this study double-layered plasma electrolytic oxidation coatings are synthesized by coating in two different electrolytes. The effects of electrical properties of first fabricated layer (in first electrolyte) on morphology and properties of total hybrid coating are investigated. First layers were applied in four different basic electrolytes consisting of sodium phosphate/citrate (PC), sodium phosphate/silicate (PS), sodium phosphate/tungstate (PT), and sodium phosphate/aluminate (PA) solutions and the second layer was synthesized in calcium acetate/sodium dihydrogen phosphate hydrate solution. The top surface and cross sectional morphology, composition and corrosion resistance properties were thoroughly studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis (M-S) methods. It was found that the morphology of the second layer depends on the morphology of the first layer, in terms of shape and size of the pores. However, XRD analysis revealed that there was no significant difference between the compositions of the oxide layers. Application of the second layer over PS sample reduced corrosion resistance from 163 to 95 KΩ cm2 while application of the second layer over PT sample led to increase of corrosion resistance from 31 to 126 KΩ cm2. The constructive or destructive role of two layers was correlated to discharge energy of the second layer which is in turn determined by the required energy for electrical charging of the first layer. The energy of electrical charging of PS and PT samples are 0.04 and 1.17 J, respectively. Therefore, plasma discharges over PS sample have higher energy, compared to PT sample, and as a result, corrosion resistance of barrier layer of PS sample drops. Electrical parameters extracted from M-S tests also verified our theory that donor density and capacitance of the first layer affect discharge behavior and intensity of the second layer. M-S results are also in conformance with evaluated corrosion resistances measured by EIS method.

Alternating Current Phosphoric Acid Etching of Aluminium

Alkaline etching is a common pretreatment process for aluminium alloys before anodising or application of organic coating. Removal of the mill-finish surface layer is performed by a simple dipping process, which also gives the matt finish normally required for aesthetic appearance before conventional anodising. Major disadvantage of alkaline treatment is the surface enrichment of the alloying and trace elements more noble than the aluminium substrate by dealloying. This may reduce corrosion resistance and deteriorate the optical properties of the surface. The purpose of this work is to investigate fluoride-free acid etching for aluminium alloys as an alternative to alkaline etching to prevent or reduce heavy metal enrichment, while achieving matt appearance. Alternating current is applied to increase the rate of etching as the dissolution rate of aluminium in acid solutions is slower than in alkaline solutions. The principle is based on anodic oxidation of the heavy metal components, as well as aluminium, during the anodic cycle and removal of the corrosion products and concentration gradients by hydrogen evolution during the cathodic cycle. The materials used for this study were hot-rolled aluminium alloy AA3105 (Si 0.3%, Fe 0.6%, Cu 0.1%, Mn 0.7%, Mg 0.3%) and extrusion AA6060 (Si 0.4%, Fe 0.2%, Cu 0.01%, Mn 0.02%, Mg 0.5%). Phosphoric acid was used as the etching electrolyte. Consumption of aluminium was calculated from weight loss. Appearance was analysed visually and by gloss measurements. SEM was used to study surface microstructure and morphology. Aluminium oxidation rate increased with increasing temperature, current density, electrolyte concentration and etching time. Intermetallic Al-Mn-Fe-Si particles were selectively and rapidly removed by 1 min etching at 80⁰C. In contrast, particles were exposed and remained on the surface by alkaline etching. Matt appearance without visible defects was achieved after 10 min etching at 80⁰C. Presence of phosphorus in thin oxide film formed was determined by glow discharge optical emission spectroscopy (GD-OES). Advantages foreseen of acid-based AC pretreatment over alkaline dip processes in practice are suitability for in-line coil processing, superfluity of the desmutting step, low gloss achievable with reduced metal loss, improved corrosion resistance and adhesion properties. Acknowledgements This work was supported by Hydro Aluminium and The Research Council of Norway.

The Effect of Friction Stir Welding on Corrosion Behavior of Ti-6Al-4V

Fusion welding can deteriorate corrosion behavior of Ti-6Al-4V alloy. However, the use of friction stir welding leads to a more appropriate corrosion resistance. In this study, the corrosion resistance of welded zones of Ti-6Al-4V alloy using friction stir welding technique is evaluated. For these purposes, the study of structural characteristics using SEM and FESEM equipped with EDS micro-analyses was conducted. Micro-hardness test was also employed to estimate the hardness of welded zones. Corrosion behavior was investigated by a potentiostat instrument. SEM micrographs, EDS and XRD analyses confirmed non-uniformity of chemical composition within the welded zones. The results reveal that the stir zone contains typical alpha and prior beta phases. Nevertheless, thermomechanical zone included equiaxed and bimodal lamellae structure. Furthermore, the presence of different types of phases and microstructure in the thermomechanical zone led to reduced corrosion resistance. The corresponding values of corrosion current density in the stir zone, thermomechanical zone and base metal were 0.048, 0.55 and 0.032 µA, respectively. Corresponding corrosion potential for these zones was estimated as −207, −110 and −157 mV. Evidently, the results show that corrosion resistance of thermomechanical zone is less than that of the stir zone and both zones have lower value than the base metal.