Feel the Burn: Welding Basics Prevent Corrosion

February 26, 2018 | Reply More

Welding basics are critical to ensuring corrosion resistance after collision repairs. We explore the corrosion implications of various welding settings including voltage, wire feed speed, stick-out, travel speed and other welding parameters.

Corrosion prevention begins with clean mating surfaces

Attempting to apply welding or adhesive bonding techniques to metal surfaces on which there is dirt, oil, wax, rust, and (in the case of aluminum) oxide film creates a high risk of joint problems. This can include development of cracks, incomplete fusion, porosity and inclusion of contaminants. Each of these problems can lead to corrosion formation and weaken the welded joint.

Dirt, paint or other coating material not adequately cleaned off prior to welding can release corrosion-causing moisture when exposed to welding heat. These contaminants may also react with elements in the base metal to release hydrogen or other corrosive byproducts of such a chemical reaction.

If panels are adhesive bonded instead of welded, the adhesive acts as a sealer to keep corrosion-inducing moisture out of the joint. By preventing metal-to-metal contact, adhesives also reduce vibration and flex-caused stress cracks that can allow corrosion to start.

Before welding or adhesive bonding, remove old weld material, dirt, adhesive or other coatings from the existing part to prepare it for joining to the new replacement component.

Aluminum Oxide

Aluminum reacts with the oxygen in the air to form aluminum oxide, a thin film that covers the aluminum surface. The oxide layer can divert heat and prevent arc penetration to the root. This can result in a joint that, instead of achieving proper fusion or bonding, may have gaps and micro-fissures that allow corrosion to form. Additionally, oxide particles can become trapped on metal surfaces under the adhesive, or in the weld puddle as it solidifies. This can cause corrosion pitting, stress fractures, and ultimately lead to cracks and metal fatigue.

BMW recommends removing oxide film with a stainless steel wire brush or special sandpaper made specifically for aluminum. The stainless steel brush is preferred because it does not leave behind impurities that can become trapped in the solidifying weld puddle, where they can lead to crack formation or can release corrosion-causing moisture.

If you use special sanding or grinding material made for aluminum, do not grind too quickly, as that may cause smearing of the oxide rather than removal. Smeared material may interfere with welding penetration, and should be filed off prior to beginning the welding process. Make sure to clean off all remaining sanding grit or dust.

The replacement component may have residue on it from lubricants used in manufacturing, or coatings applied for protection during shipping and warehousing. The hydrocarbons in lubricants and some coatings can break down during welding and release hydrogen into the molten weld puddle. As the weld solidifies, this hydrogen comes out of solution and coalesces into bubbles, leading to porosity and potential entry points for corrosion.

Clean the mating surfaces with BMW cleaning agent R1. Do not use compressed air to clean off sanding or

 

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Category: Structural & Body, the bimmer pub

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