After 6063 aluminum extrusion undergoes anodization, it possesses excellent corrosion resistance and decorative qualities. In recent years, with the growth of the national economy and the improvement of living standards, the use of aluminum alloy doors, windows, and curtain walls has become increasingly prevalent.
However, after some time, many aluminum alloys develop corrosion defects of various forms on their surfaces, among which spot corrosion is quite common. This significantly impacts the performance and aesthetic appeal of the aluminum extrusion.
To effectively improve the surface quality of aluminum extrusions and control spot corrosion, it is imperative to conduct a thorough and detailed analysis of the defect.
This article examines spot corrosion on the surface of 6063 aluminum extrusion after anodization, analyzing its nature, causes, and formation mechanisms, and explores the key factors leading to spot corrosion.
1. Analysis of the Nature of Spot Corrosion
As we know, the composition of the 6063 aluminum extrusion we use is designed to ensure that the Mg element forms the strengthening phase Mg2Si sufficiently.
Generally, when preparing alloy compositions, the Si element is intentionally allowed to be slightly excessive. As the Si content increases, the alloy’s grains become finer, and the heat treatment effect is better. However, on the downside, excess Si reduces the alloy’s plasticity and worsens its corrosion resistance.
Studies have shown that excess Si can not only form a free-state Si phase but also form α phase (Al12 Fe2Si) and β phase (Al9Fe3Si2) with the matrix, resulting in the presence of free-state Si phase, α phase, β phase, and anode phase Mg2Si particles in the aluminum alloy.
α and β phases have a significant impact on the corrosion resistance of the alloy, especially the β phase, which significantly reduces the alloy’s corrosion resistance.
The main components of the residues at the spot are free-state Si phase and AlFeSi phase. Chlorine elements are also found to be absorbed at the residue site, indicating that Cl- is involved in the corrosion process.
The zinc content in the corrosion area is much higher than in the matrix, suggesting that impurity elements of zinc in the alloy also participate in the corrosion process.
During the anodization process, the anode phase Mg2Si is the pitting source of the alloy. During alkaline washing of anodized alloys, Mg2Si particles dissolve preferentially, forming pits where magnesium dissolves into the solution and silicon remains on the aluminum alloy.
When pits gather on the grain, it darkens the color of the grain. Silicon is difficult to remove in the sulfuric acid process, so the silicon content at the bottom of the spot corrosion pit is higher than in other areas.
2. Analysis of Pitting Corrosion Causes
The main factors influencing pitting corrosion include the temperature and duration of alkali washing in the pretreatment process, the content of Zn, Fe, Si elements in the alloy composition, and the extrusion state of the alloy.
Among these factors, the extrusion state plays a crucial role. It relates to the distribution of corrosion-affecting elements such as Zn, Fe, Si, and the precipitation locations of intermetallic compound particles.
In areas with broader extrusion striations, the distribution of pitting corrosion has a noticeable directionality. This is because these regions encounter higher resistance during extrusion and stress tends to concentrate here. The metal lattice in these areas undergoes severe distortion, becoming areas of localized high free energy.
These areas preferentially nucleate during subsequent recrystallization processes to reduce interfacial energy and attain stability.
Here, the grains not only grow abnormally large but also compounds such as Mg2Si anodic phase, free Si, FeSiAl, FeAl3 cathodic phases preferentially precipitate, setting the stage for subsequent pitting corrosion.
Due to the above reasons, areas impoverished in silicon and iron elements appear near the grain boundaries where free Si, FeSiAl, FeAl3, and other intermetallic compounds precipitate. This area, nearly pure aluminum, has a negative potential and acts as an anode. It forms a microcell with the intermetallic compound (which acts as a cathode).
Under the influence of corrosive mediums, the Si, Fe impoverished areas (anodic phases) surrounding the cathodic phase (e.g., free Si, FeSiAl, FeAl3) in the microcell dissolve preferentially, as does Mg2Si.
The dissolution of Al surrounding the anodic phase results in corrosion pits with residues, while the dissolution of the anodic phase leads to corrosion pits without residues. As corrosion conditions continue to worsen (like rising temperature, prolonged alkali washing), the base Al continues to dissolve, and the corrosion pit deepens.
The surface morphology then exhibits both corrosion pits with residues and those without, resulting in the aforementioned pitting corrosion.
3. Analysis of Pitting Corrosion Formation Mechanism
6063 is an Al-Mg-Si alloy where Mg2Si is the sole age-hardening phase. To enhance alloy strength, Si content is often made excessive during production, leading to the formation of free Si and FeSiAl phase particles.
Under inappropriate extrusion techniques and non-standard heat treatment, these particles may coagulate (or segregate) at grain boundaries with FeAl3, Mg2Si particles, forming pitting sources.
According to corrosion theory, the aluminum anode surrounding the cathode particles will corrode preferentially.
Generated Al3+ diffuses towards the cathode, while OH- in the solution diffuses towards the anode. Eventually, white flocculent Al(OH)3 precipitates at the cathode-anode interface. Upon drying, it forms white spots on the aluminum surface, resulting in what’s known as pitting corrosion.
The corresponding chemical equations are as follows:
- Al → Al3+ + 3e- (Anode)
- Al3+ + 3OH-→ Al(OH)3 ↓ (Cathode)
4. The Impact of Active Elements
Accelerating Effects of Zn Element
Zinc, when dissolved in aluminum alloys, promotes grain corrosion through a “dissolution-redeposition” process. Cathodic particles such as zinc or iron deposited on the alloy surface, as well as high-potential dissolvable substances like FeSiAl and free silicon, can effectively accelerate the reduction of dissolved oxygen, catalyzing continuous corrosion expansion and deepening.
During the alkali washing process, the Zn element dissolves with Al and resides in the alkaline solution in the form of Zn(OH)4^2- and Zn(OH)-3.
Because the potential of Zn (-0.76V) is more positive than Al (-1.67V), when the concentration of Zn ions in the alkali solution reaches a certain value, Zn selectively deposits on the residue in the corrosion pits, leading to an abnormally high concentration of Zn.
On the other hand, due to the significant potential difference between Zn and Al, the corrosion current in the micro battery is high. Cathodic particles Fe, Si in the depleted zones (essentially pure aluminum) dissolve quickly, which eventually manifests as pitting corrosion.
Activation Effects of Cl-
As an external factor, Cl- is highly sensitive to pitting corrosion, with the ability to induce and exacerbate pitting. Research found that Cl- in the degreasing acid adsorbs at the defects of the passivation film, and penetrates the film to adsorb on the substrate.
The aluminum elements here dissolve rapidly due to activation, thereby destroying the passivation film and forming a galvanic cell structure. Under acidic conditions, the localized corrosion current is high.
At this point, Cl- reacts with the dissolved Al3+ as follows: Al3++Cl-+ H2O→AlOHCl++H+. This reaction further strengthens the acidity of the solution, worsening the corrosion condition.
As Cl- concentration increases, the complex reaction proceeds to the right, significantly increasing the active points on the passivation film. These points dissolve preferentially during the subsequent alkali washing process, leading to severe pitting corrosion.
Promoting Effects of pH
When the pH of the rinse water is less than 2 or greater than 4, pitting corrosion rarely occurs. During the process where the color of the grains darkens from gray to black, the pH in the rinse tank plays a promoting role.
When the pH of the rinse water is greater than 4, the passivation film formed on the aluminum profile surface is relatively complete and dense. The adsorption, activation, and destruction effects of H+ and Cl- are greatly weakened, hence there is little or no corrosion.
When the pH is less than 2, the aluminum profile surface is in an active dissolution state with no passivation film formed, thus pitting corrosion does not occur.
5. Conclusion
The pitting corrosion in 6063 aluminum profiles is caused by the segregation and coarsening of the anodic phase Mg2Si in the aluminum alloy, while the impurity element Zn in the alloy and Cl- and pH in the solution accelerate the occurrence and development of pitting corrosion.
It is necessary to appropriately adjust the mass ratio of magnesium to silicon in the alloy, avoid excessive silicon content, and arrange the aging system reasonably to prevent the agglomeration of Mg2Si particles, thereby avoiding any impact on the corrosion performance of the aluminum profiles.
Controlling the trace element Zn in the alloy and the concentration of Cl- and pH in the treatment process reduces the negative impact of active elements.
