A lintel is a horizontal steel beam support placed over a wall opening to carry the weight of the masonry placed over it. In most situations, the lintel above your windows and doors will be steel angle iron, or I beam. Either due to age, improper rustproofing or weep chord and flashing placement during installation, lintels will begin to rust. Rusting lintels can expand to become much thicker then their original size. The expansive forces can shift structures. Diagonal “step” pattern cracks, which can be found on either end of the window or door opening, are a sign that your lintels are rusting.
In majority of situations, rusting lintels must be replaced.
Unfortunately, unless you go to the trouble of having the mortar custom made to match, the repair will stick out like a sore thumb forever. Hire a Shamrock Tuckpointing for a perfect match.