The Mg-Si Balance Behind Profile Corrosion
Aluminum profiles are chosen for architectural and industrial work because the metal forms a stable protective oxide and holds its appearance over long service. Corrosion that does appear on a finished profile is usually not caused by the weather but by how the alloy was made. The magnesium and silicon in a 6000 series alloy are intended to combine into magnesium silicide, Mg2Si, which is the phase that provides precipitation hardening. When silicon is present in excess of the stoichiometric requirement, the surplus cannot enter the Mg2Si phase and instead appears as free silicon or as insoluble ternary compounds. Those phases tend to gather at grain boundaries, where they weaken the boundary and become the point at which corrosion first initiates, because the boundary is now electrochemically different from the grain interior.
EN 573-3 sets the composition window that keeps this under control. For 6063 the magnesium range is 0.45 to 0.9% and the silicon range 0.20 to 0.6%, with iron limited to 0.35%, copper to 0.10% and chromium and manganese to 0.10% each. For 6061 the silicon range is 0.40 to 0.8%, magnesium 0.8 to 1.2% and copper 0.15 to 0.40%. Buying to those limits, and verifying them by spectrographic analysis of the billet, is the first line of defence.
Melting and Casting Practice
A specification-compliant analysis on paper can still deliver an inhomogeneous billet. During melting and holding, the silicon and magnesium must be distributed through the whole melt, and if stirring is uneven or too short the result is localised enriched and depleted zones. The billet that is cast from such a melt inherits a non-uniform structure, and the finished profile shows it as patchy corrosion resistance, uneven colour after anodising and abnormal grain growth. Free silicon in the aluminium matrix is particularly damaging because it both lowers corrosion resistance and coarsens the grain, which in turn increases the depth of the anodic film required to cover the structure and reduces the surface gloss. Holding time, melt temperature and grain refiner addition all belong under the same control chart as the composition analysis.
Extrusion and Ageing Parameters
Parameters set at the press decide whether the magnesium stays in solution and later precipitates as useful Mg2Si or is lost to segregation. Billet preheat temperature that is too high, an extrusion speed that is too fast, insufficient cooling at the press exit, or an ageing temperature and soak time outside the agreed curve all push the material toward silicon segregation and dissociation, so part of the magnesium and silicon fails to form Mg2Si and free silicon remains. The counter measures are a controlled billet preheat profile, a press speed matched to section complexity, quench rate verified by hardness survey rather than assumed, and an ageing cycle held within narrow limits, typically a controlled soak with a recorded chart for every load.
Surface Treatment and Service Environment
An anodic film is only as good as the metal beneath it. A profile with coarse grain and segregated boundary phases requires a thicker film to cover the structure adequately, and thin or porous films allow the boundary attack to continue under the coating, producing streaking and black spots. GB/T 5237.2 and EN ISO 7599 set film thickness classes: AA10 requires a 10 µm average with an 8 µm minimum, and AA15 requires 15 µm average with a 12 µm minimum. Seal quality matters as much as thickness, because an unsealed anodic film remains permeable. In service, chloride and industrial sulphate atmospheres accelerate attack at any weak point, and profiles in contact with dissimilar metals such as stainless steel fasteners can suffer galvanic corrosion unless an insulating washer or a compatible fixing is used. Runoff from fresh concrete and from timber treated with copper-based preservatives is another known aggressor on exterior profiles.
Verification Before Purchase
Verify the alloy by spectrographic analysis against EN 573-3 rather than relying on the label; request the heat number and the corresponding analysis for each extrusion lot. Check mechanical properties against EN 755-2 with a tensile test on a specimen cut from the profile, and apply a hardness survey across the section to expose an inadequate quench. Confirm surface treatment by eddy current thickness measurement on the visible face, seal quality by the acidified dye stain or admittance test, and coating adhesion by impact and bend tests. Salt spray testing under ISO 9227 with a defined acceptance time provides an end check on the anodic or organic coating. All results belong on a mill test certificate to EN 10204 3.1, and the profile should carry an alloy stamp and a heat number on each bundle so the material can be traced back to the billet.
Frequently Asked Questions
Q: Is surface corrosion on an extruded profile a manufacturing defect?
A: In most cases it is. A correctly cast, extruded and aged 6000 series profile with an adequately thick and properly sealed anodic film resists normal atmospheric exposure, so early corrosion points to alloy control or process control failures.
Q: What causes white or grey spots after anodising?
A: Segregated phases and free silicon at the grain boundaries dissolve or react differently in the anodising bath, so the film forms unevenly. The underlying cause is billet quality rather than the anodising line.
Q: Can a thicker anodic film compensate for a poor billet?
A: Only partly. A thicker film covers more of the structure and improves appearance, but boundary phase attack can continue beneath the coating, so film thickness should not be used instead of composition control.
Q: How should profiles be stored and handled on site?
A: Keep them dry and off the ground, remove protective film within a reasonable period to avoid adhesive transfer, avoid contact with fresh concrete and copper-treated timber, and isolate dissimilar metals with a suitable washer or gasket.
Q: Which tests confirm the alloy is what was ordered?
A: Optical emission spectrometry for composition against EN 573-3, tensile testing against EN 755-2, and a hardness survey across the section to confirm the extrusion quench and ageing cycle were applied correctly.
