What might be a good article about colonialism and racism in Heart of Darkness?
There are certainly many articles on colonialism and Joseph Conrad's Heart of Darkness that can be tracked down via library databases and even Google Scholar. Published in PN Review in 2014, Chris McCully's "Culture, Civilization, Conrad" is likely one of the most recent examples. Michael Lackey's "The Moral Conditions for Genocide in Joseph Conrad's Heart of Darkness" published in the peer-reviewed journal College Literature would also be a more recent work worth considering. There is, in fact, a peer-reviewed journal dedicated to Conrad's work titled Conradiana that has many articles on Heart of Darkness, including one that you might find useful titled "Conrad and Ambiguity: Social Commitment and Ideology in Heart of Darkness and Nostromo" by Nursel Icoz. The article that is considered to be the most significant work on Conrad and Heart of Darkness, however, would be Chinua Achebe's "An Image of Africa."
Achebe's article takes issue with claims that Conrad's work is meant to be taken as ironic. Achebe notes that F.R. Leavis has pointed out that Conrad consistently draws a contrast between the reserved nature of the Englishmen and the "frenzy" of the African people. Therefore, if we are to read Heart of Darkness as ironic, then it would be said that Conrad is offering a condemnation of Marlow (the protagonist) and his perspective on Africa and the African people. Instead, Achebe asserts that "for Conrad, things (and persons) being in their place is of the utmost importance" and notes either latent or obvious racist tendencies within Conrad's prose.
For more, see the reference links below.